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This question is inspired by several similar questions on this site.

The scenario: at one moment, all people above the age X disappear without a trace. How low this X can be that human civilization survives?

Obviously, 1 year old is too young, and 18 is arguably too old.

I set the following criteria of success:

  • Language is kept (which may change with time, it's Ok);
  • Tech level above stone age is maintained (at least in some communities). Scavenging is Ok for as long as people are able to scavenge; no mining or smelting is needed up until that point;
  • Sufficient artifacts and knowledge from our current civilization survives, so, eventually, younger civilization can utilize it for a "reboot".

Children don't have to maintain world in order after the event. Any level of chaos, regression and massive dying is acceptable if the rules above are satisfied.

P.S. Extra bit of information - there are apparently more than a few children under the age of 18 (likely thousands, if not tens of thousands) who are familiar with Survivalism. A proper "failure" scenario should mention how all of those children will be eliminated, or isolated for the rest of their lives.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch Sep 2 '18 at 6:18
  • $\begingroup$ Note the the question asked in the text is entirely different from that asked in the title. $\endgroup$ – RBarryYoung Sep 3 '18 at 4:57
  • $\begingroup$ @RBarryYoung how do you think this question should be titled? $\endgroup$ – Alexander Sep 3 '18 at 7:40
  • $\begingroup$ are you willing to accept huge losses in the kids, like most of the kids in the city when the supermarkets run out of food? $\endgroup$ – WendyG Sep 3 '18 at 8:42
  • $\begingroup$ @WendyG Yes, as I said, any loss of life is acceptable as long as someone somewhere keeps the civilization alive. $\endgroup$ – Alexander Sep 3 '18 at 20:47

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I'm not convinced that 18 is too old at all; in point of fact, if ALL the 'adults' left at once, society would be ruined in many different ways.

What we call civilisation is really a very delicate balance in this world. Let's start with something as simple as food distribution. Without people who know how to drive trucks, most of humanity will starve because it lives in cities and its food is produced on farms. Even if an 18YO knows how to drive a truck, they're not experts in logistics, planning, economics, etc. That's even assuming there's enough fuel to power the trucks, now that all the workers who actually know how to operate oil rigs and refineries are gone.

Let's assume that this mass extinction is really a mass disappearance, and there aren't a truckload (no pun intended) of bodies to dispose of before they generate disease, the real problem here is that civilisation, society, and all the other words we use to describe the framework of human social and cooperative interaction is NOT based on knowledge alone; it's also based on experience.

You've just lost all the experience that would under normal circumstances mentor the younger generation through the awkward transition of knowing something theoretically to actually understanding how to do it.

Any mechanic will tell you that knowing how to fix a car isn't as simple as reading a book on how engines work. The value an older mechanic brings to the table is all that experience of knowing what that knocking sound on the left side of VWs really means, and what to do to fix it. Similar scenario with doctors, and other critical professions.

Engineering is another example; knowing the math is one thing. Knowing how to design a building by knowing how people think, how they work, how they use the various elements of the building, what they care about - that's decades of experience.

We often make fun of the older / younger generation (cross out that which does not apply to you) and we talk about how the old world thinking is no longer relevant, and how these new whippersnappers come in with their fresh ideas and no understanding of how the world really works, and we're both right. At the same time, we're both terribly wrong.

The young in society are able to surpass the old precisely because the old are sharing the knowledge and experience they've accumulated with the young. That gives the young a 'leg up' on things, allows them to devote more of their time to refining and improving on what's already known rather than learning it all over again.

What you're doing is effectively disrupting that cycle. Assuming all the young folk don't die off in the short term from starvation or disease, the society that would remain would go back to a tribal nature, at least for a time. Cars and other tech may not last through extended periods without this young generation taking up the mantle across a very wide range of industrial fields, and if even one of those fails, it's possible society collapses, at least for a few generations.

Bottom line is that there are very few industrial and technical pursuits in our society that are extraneous, or even easy to learn without experience of others being passed on through the generations. Your society not only has to keep all that going without the experience, but then has to learn to pass that on in their own turn without the benefit of having received it themselves.

Your child society is in more trouble than I think you suspect.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch Aug 31 '18 at 19:46
  • $\begingroup$ "Any mechanic will tell you that knowing how to fix a car isn't as simple as reading a book on how engines work." Maybe, but, anecdotally—I've just started getting into fixing cars using "books" (YouTube videos), and I'm finding it surprisingly easy. $\endgroup$ – AmagicalFishy Sep 3 '18 at 2:10
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    $\begingroup$ @AmagicalFishy It's true that some people will learn that way, but I'd also argue that YouTube isn't the same as books; it's a recording of the experience I was talking about, someone 'showing' you how to do it. Without that expertise available, I'd also point out that the archive of YouTube videos won't be available to the children remaining. If you can learn to be a good mechanic by reading an old Gregory's manual and learning from that, then I'm more likely to be convinced. (Gregory's manuals used to be the bibles for maintaining specific car models before the internet) $\endgroup$ – Tim B II Sep 3 '18 at 2:16
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    $\begingroup$ Also, YouTube wouldn’t be accessible for about a half-dozen reasons. So ... $\endgroup$ – RBarryYoung Sep 3 '18 at 4:49
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    $\begingroup$ Re:bodies, adults might not be the only possible source of bodies here. It's a grim thought, but even if their bodies disappeared without any trace, just imagine all the sole infant or toddlers left in their home with no way to take care of themselves... $\endgroup$ – votbear Sep 3 '18 at 6:27
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Children in more traditional, agriculture based societies have much better chances of survival. Especially in areas where families still live almost self-sufficiently on farms and children are involved in the farm work.

For example, among the Amish: "Until the children turn 16, they have vocational training under the tutelage of their parents, community, and the school teacher."

And child labour is still common in Africa: "Agriculture alone employs more than 30% of all African children aged 10–14."

Based on this, I'd estimate that the limit would be somewhere between 10 and 16 years of age. Of course living on a self-sufficient farm is demanding, and children would have hard time surviving on their own. But there is enough of such families that even if some don't make it, some will. It helps that there are usually many siblings, so they don't get isolated when adults disappear.

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  • $\begingroup$ Looks like the more complex society the greater the fall and the other way around. Often children (mostly girls) in agriculture societies learn to cook at a young age (7-10), that would also help against possible disseases. $\endgroup$ – Mixxiphoid Aug 31 '18 at 11:50
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    $\begingroup$ This. Rural Africa, and most of Asia are all going to be 'fine' (relative term). $\endgroup$ – Korthalion Aug 31 '18 at 14:40
  • $\begingroup$ In many countries, pre-teens know a lot about how to get along. In others, many teens are clueless, but there are still a sizable percentage that are creative and resourceful. $\endgroup$ – WGroleau Sep 2 '18 at 3:53
  • $\begingroup$ Im not sure child laborers have the right exposures to reboot society, granted they can survive but so can a lot of other kids since the human population would be culled by like 75% thereby leaving a lot of resources behind. I just don't see an African shoe making child being able to dig through the books needed to bring back any kind of technology. $\endgroup$ – anon Sep 2 '18 at 21:45
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About 35

And even then it would be traumatic.

Civilisation is the accumulated knowledge of our need to work cooperatively to survive. It's a set of skills and knowledge accumulated over centuries, passed down from one generation to the next by a combination of education and in-work training.

To be able to maintain your technological level you need the infrastructure, that requires an active workforce who have the skills and knowledge to operate it. At around age 35 a graduate has 10-15 years of practical knowledge and some seniority. You've killed off the government, much of the bureaucracy and any senior management outside the tech industry.

You've also killed off much of the legal profession, large swathes of your food suppliers and a few other industries that you probably haven't thought of that define civilisation, like art historians for example.

What's more interesting is you've still killed off many of the people who have the skills needed to allow you to maintain any technological level without the major infrastructure of the power stations and power distribution networks. People like blacksmiths and farriers who know how to work metal by hand.

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  • $\begingroup$ I’d like to know why this was downvoted, given that this is obviously the correct answer. $\endgroup$ – Konrad Rudolph Aug 31 '18 at 12:04
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    $\begingroup$ I didn't downvoted, because I see it as to much opinion based. However, it seems to be like the answerer is assuming a rather higher level society as required than "above stone age" which was asked about. Instead, he seems to insist on keeping the current society running without an inordinate amount of disruption. This'd be a setback only in the most advanced reaches of science. $\endgroup$ – Jacco van Dorp Aug 31 '18 at 12:15
  • $\begingroup$ There are two problems, one is that I've ignored the spirit of the question, that being that only children survive, the other is the all or nothing aspect of technology. The children have nothing, not even the basics of blacksmithing or casting, so you have the choice of keeping it going as much as possible or sending them straight back to the stone age as soon as the power goes out. $\endgroup$ – Separatrix Aug 31 '18 at 12:39
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    $\begingroup$ I didn't downvote it, but I too see this answer as opinion-based. IMHO we have so much products and materials around that "sending them straight back to the stone age" seems almost impossible. $\endgroup$ – Alexander Aug 31 '18 at 20:07
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    $\begingroup$ @Separatrix if you define a technology as having an active full cycle, then yes, but this is not how I defined it for the purpose of this question. Ex: if every mine on the planet is shot down for a month, does it mean we are in stone (or below stone) age for that month? $\endgroup$ – Alexander Aug 31 '18 at 21:04
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Responsibility

The other answers here concentrate on technical know-how so much that they completely miss another factor: Responsibility and self-control. Now that makes me sound old, hu? However, if you think about it, even grownups without responsibility and self-control can easily fail their jobs. I would say childhood is by definition not there yet, it's about achieving those traits. It comes from within, but it also comes from parents teaching their values.

Scavenging

I think we all agree that there will first be a period of scavenging. Mixxiphoid commented* on your question

If you remove everybody older than 18 from Japan you removed about 80% of the population. In most of South-America it is the other way around.

That makes a huge difference for scavenging. With children being 1/5 of the population, in the supermarkets etc. there will be non-perishable food corresponding five persons available to one kid. If this is literally the other way around, that's only non-perishable food for a bit more than one person per kid. For long term survival, farming is important, but the non-perishables determine how much time the kids have before they need their first harvest.

Also note that there's quite the trade-off here: If your age X is high, you will have more know-how, skills and responsibility, but also less food per person, so you will need to start to farm earlier.

*I've got comments here that those numbers are not correct, so don't take them that way. However, they don't need to be correct to illustrate the concept of the kid-to-food ratio.

Farming

Farming is important for long term survival, but it's also a long-term process. The problem I see here is that if you're running out of food, it's too late to start agriculture and there's not much of a second chance. It probably averages around half a year of planning, from seeding somewhere in spring to harvesting somewhere in autumn. We all remember how back in the days, the next Christmas was just "very far away". You need to not only understand the concept of a calendar or at least the 4 seasons, but also a way to determine where you currently are on that timeline. You also need the (let's call it) discipline to not forget things that have to be done less than daily.

However, I think "the concept of seasons" is the easier one. What will be more difficult will be to get through a work day. If you get "Are we theeeeere yeeeetttt" after an hour long car ride, I doubt that we get a decent workday of boring, repetitive farm work out of that kid. Especially purely internal, without the impulse control that grownups are. Again, there's enough examples of such behaviour from adults, even with the emotional support of other adults.

Japan - the Mekka of saving society with children

Given these musings, Japan actually seems like a prime candidate here. They have big cities that will have a lot of non-perishables stored and they have a really good "age multiplicator" on that value. They have arable land - and with rice also an efficient locally growing food crop. Seeing how children clean their classrooms at the end of the day etc., it also seems like they have a leg up on what we called "discipline" before.

Some farmer's children are already helping out on their farms, so they have the know-how and will be able to go on farming. However, apart from know-how and discipline, they also need the idea of continuing to farm. While there's still a lot of non-perishables, they need to understand that the situation is dire and they need to farm. User anon provided this information about cognitive development in their answer, which lists "Begins to think long term" under the header "Middle Adolescence", which would be around age 15.

That being said, "dramatic events" could have dramatic effects. For example, if many kids die early on from something else than hunger, the leftover non-perishable food would be able to sustain the survivors longer, pushing the number down. However, they will need to be at least of reading age so that they can study the old texts. If nobody can read anymore, it will probably be too difficult to decipher a for all reasons foreign script, especially when daily life is about fighting for survival.

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    $\begingroup$ I was actually optimistic with the 80% figure. It is likely a bit higher, toward 85%. $\endgroup$ – Mixxiphoid Sep 2 '18 at 8:30
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    $\begingroup$ If you remove everybody older than 18 from Japan you removed about 80% of the population. In most of South-America it is the other way around. It is definetly not. You would remove more than 70% of the population of South America. For Brazil, by far the most populous country of the region, you would reduce the population from 200,000 to about 50,000 - by 75%. Indeed, I don't think it would be "the other way round" even in Subsaharian Africa or Southern Asia. $\endgroup$ – Luís Henrique Sep 2 '18 at 12:43
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    $\begingroup$ @LuísHenrique I believe you mean 200 millions to 50 millions, I'm sure there's more than 200,000 people in Brazil :P $\endgroup$ – Adrien Sep 3 '18 at 10:15
  • $\begingroup$ @Adrien - Yeah... such is the power of zeroes. $\endgroup$ – Luís Henrique Sep 3 '18 at 12:02
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    $\begingroup$ Another issue: scale. Going back to being a teenager, I'd roughly know how to plant some kinds of crops - but I really wouldn't know how many I'd be required to plant in order to live from one harvest to the next. (Though I guess this could be alleviated a bit if the kids got some of those mass-scale agricultural vehicles running for a bit, then they might be able to get too many crops for the first harvest, at least until fuel runs out). $\endgroup$ – hoffmale Sep 7 '18 at 16:35
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if these are the criteria:

I set the following criteria of success:

  • Language is kept (which may change with time, it's Ok);
  • Tech level above stone age is maintained (at least in some communities);
  • Sufficient artifacts and knowledge from our current civilization survives, so, eventually, younger civilization can utilize it for a "reboot".

Then I somewhat arbitrarily assert based off of cognitive development of children that the minimum age needed to achieve the above falls roughly between 12-13 years of age.

Society will collapse horrifically from the event with lots of children dying, however, 12 year olds generally have obtained enough skills in literacy and mathematics to re-learn lost knowledge from leftover books. Moreover, key cognitive changes occur at that age range allowing them to adapt to cognitively adapt to those changes. Children younger than that would probably adapt more primitively with a higher chance of loss of knowledge........

This isn't perfect because a lot of it is individual and situational driven. Theoretically, if a child as young as 2 managed to learn literacy and survive the apocalypse they could reboot society.

Not to mention overcoming the literal "Lord of the Flies" scenario incurred by the loss of authority, however that will happen even if the age cap is 18.

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    $\begingroup$ I upvote this however I would extend the bottom limit. Many 10 yo kids know how to operate things so they will be able to create simple tools, carve wood and stone etc. I guess even 8 years old or younger kids who work with their parents would be able to do that and the mere knowledge that it is possible to do and was useful for the adults then they were still here will help them develop more abilities later from the leftover books. Some will be more brilliant than others and those will lead catching up. So I would say even if only 6 yo survives it's likely they will reboot the civilisation. $\endgroup$ – Ister Aug 31 '18 at 7:33
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    $\begingroup$ @Ister I agree, the reason that invention took so long up to this point is that certain concepts just weren't there. Once you know something is doable it makes it a lot easier to 'rediscover' it. $\endgroup$ – Korthalion Aug 31 '18 at 15:10
  • $\begingroup$ I would argue most 10-13 years old don't know how to produce meals beyond breakfast cereal. $\endgroup$ – WendyG Sep 3 '18 at 8:37
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    $\begingroup$ I think what a lot of answers miss is that it is much easier to re-create something than it is to create it. We'd be in the agriculture age within 10 years or so. Horses and plows. Heck, even my 6 year old knows that to make steel you need iron and carbon (he learned it from a video game.) I'd say there is an even chance that after 100 years we'd be back to at least the industrial age. We learned it once, we can do it again. Humans are resilient, curious, and tenacious. $\endgroup$ – Tracy Cramer Sep 4 '18 at 16:21
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It jumps off the page that the death of (say) 99% of the population would not mean that any of your three criteria had failed.

I think criteria 1 and 3 would be met if 6-year-olds were to survive. Some of them, especially from subsistence cultures, would have some idea about farming. Even if they couldn't do it, at first, some of them would find enough food stocks to to survive until they were old enough to figure out how to farm. Most would not, but it only takes a few. Some 6-year-olds can also read and know about dictionaries. Libraries would survive and would still be there for people to use to re-learn.

Criterion 2 is tougher. I am assuming that a temporary loss of technology, followed by re-learning even within a few years, does not qualify as "maintained". I also think you need significant age to maintain knowledge of manufacturing. I'm guessing you'd need age 15 or so for above-stone-age tech to be maintained in "at least a few communities". The electric grid would fail, but there are a lot of above-stone-age technologies that don't depend on it, i.e. metalworking, paper, non-electricity-based printing, sailing, mechanical watchmaking, and college-level physics, chemistry, biology, and math, all of which would be would understood by a few bright children. With 15-year-olds, I'd assume that tech would initially drop to the level of 1600 or so -- more in some areas and less in others. That is still above-stone-age.

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    $\begingroup$ Thank you, William. In this case "maintaining technology" does not mean having to maintain full manufacturing cycle, it means only "make effective use of technology". For example, 6 year old has a knife and knows how to use it - we are good. If his knife had dulled, but he can sharpen it - we are still good. But if the knife had dulled, and he can't sharpen it or find a replacement - that's when we lost it. $\endgroup$ – Alexander Aug 31 '18 at 20:01
  • $\begingroup$ We have actually rebooted the societies a few times in history. Usually this has set us back by about a 500-1000 years. Even then the rebuild wasnt usually done by reading old texts. But by importing it from some where else, atleast to a part. So while i think your answer covers the bases. I also think thats it, we blew our last chance. See never before in history has energy reserves been so depleted. So not only is it harder to do it this time but you wiped the knowledge and will to do this. Remeber as the first generation dies, so dies the ambitions to preserve the means to bootstrap. $\endgroup$ – joojaa Sep 1 '18 at 21:07
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About 15

I once met a young man who was 15 years old in a country with a poor economy (Guatemala). He was working really hard at a variety of things, so I stopped to chat with him. He was building a factory for casting aluminum pots. He had already build one set of molds, and he built his fires from biofuels his little brother gathered in the area. He was taking care of his little brother and sister without any adult supervision.

He had figured out how to make these pots on his own, developed a marketing plan, and was expanding his line. There was a few other street urchin boys who were hanging around, trying to get a job.

And in poor countries, a 15 year old adult, or even younger is not that uncommon.

Even more, in rich countries, children in neglected and abusive homes start functioning as adults earlier than age 10. For example, I knew how to drive, avoid the police, cook, grocery shop, balance a check book, and repair a furnace by age 11. I surely could have figured out how to make pottery and stone tools; much easier than many tasks that I already knew. However, 11 is too young to have the social maturity to interact with a community the way the 15 year old Guatemalan (as I mentioned above).

Edit:

If you want to learn more about high-functioning youths, check out the movie Turtles Can Fly. In my opinion, a very realistic film.

The more I think about it, I have known many 15 year olds who survived without adult help very adverse situations - rape, homelessness, addiction, etc. The idea that 15 year olds lack the ability to have emotional maturity, as other answers say, is wrong.

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    $\begingroup$ Interesting contribution. I'd like to point out that the frontal lobe in the human brain is only fully developed at age 25(ish). And this lobe is associated with social behaviour (among other things). So, while a very young adult may learn surprising skills, his personality an social behaviour keep on developing until age 25, for biological reasons (and later for other reasons, but that's a different matter) $\endgroup$ – Burki Sep 3 '18 at 12:43
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The age cutoff would need to vary depending on the child's experience, proximity to resources, and the nearby population of other survivors. Here is a plausible scenario that could work.

Everyone will need to be fed. Farm children are often given chores, starting at around age 8-10, with increasing responsibility as they get older. A large enough family should have an eldest child who knows enough about the operation of the farm to keep it reasonably self-sustaining for the near term. Although a few 12 year-olds might be able to take on this role, a more reasonable age would be 14 or 16.

This answer to a related question makes a great point about military campuses as a means of preserving civilization. The cut-off age for this group would be at least 20, although 22 or 25 would be better. Among their advantages:

  • They have an existing structure to make and follow orders.
  • They have better access to technologies for self-sufficiency (e.g. weapons, diesel vehicles) than your average city-dweller.
  • Although they are not experts in medicine, engineering, logistics, or law, many cadets have basic training and an interest in these fields, providing a good starting point for preserving or rebuilding technology.
  • Some of them will have the foresight to plan beyond survival, and work toward the retention of existing technology and the long-term re-development of civilization.

These two populations could mutually assist each other in a concept I call "the citidel".

  • Farms would grow crops needed for food and raw materials.
  • The military would develop and produce bio-diesel fuel, needed for farm equipment and military vehicles/generators.
  • Simple goods such as clothing could be made on farms. More advanced goods such as machinery parts would be handled by the military, both by scavenging and by manufacturing.
  • The two populations would trade goods with each other.
  • The military would provide transportation of goods and people.
  • The military would provide law and order, much like a county sheriff.
  • The military would provide other specialized services such as medical care, equipment repair, and dispute resolution.
  • The youngest children would be raised on the farms. They would receive an elementary education there.
  • During the farming off-season, persons above a given age could be conscripted into projects such as road repair.
  • When farm children get old enough, they would have the option of leaving the farm, joining the military, and receiving higher education.

It should be noted that West Point is within marching distance of productive farmland.

Furthermore, the survival of these two particular groups could be related to the cause of the disappearance of adults. For example, if adults were being abducted by aliens, these two groups would have a better chance to resist (access and training with weapons) than your typical city dweller.

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It's way harder than you think. Maybe impossible.

I'm going to tackle this from a different direction. What is the minimum age to meet each of the three requirements?

Language is kept

This is an easy one. Language is extremely useful, and children acquire it early. Probably any group of children above five years or so of age will keep some sort of language.

Tech level above stone age is maintained

This is where things get a bit tricky. What exactly is tech level above stone age? Do they have to actually be able to make their own tools, or can they simply re-purpose existing scrap. It's way easier to fashion an extra shovel into a passable pitchfork, or an extra pitchfork into a passable shovel, than to make either one from scratch.

So if your goal is to have them be able to salvage portions of existing objects for their own purposes, I doubt you need to increase the minimum age very much. Certainly by the time children are 7 or 8, they'll be familiar with the basic premises and purposes of basic implements like the hammer, the plow, the saw, the anvil, etc. Human ingenuity is a pretty powerful thing, so they should be able to survive for generations without ever even needing to know that metal ultimately comes from the ground.

If you want them to be able to produce new goods "from scratch", that's a lot trickier. Knowledge of smelting, for instance is almost certainly lost. Not only do very few children (or adults, for that matter) know how to recognize the proper ores and build sufficiently hot fires, but the bigger issue is, none of them will actually care. If any significant portion of the population disappears, there will be so much scrap lying around that for probably at least a generation or two it will be far easier to scrounge what you need than to make it yourself.

Minimum age for scrounging: 7-8

Minimum age for producing: 30-40 (The issue isn't that you need to be 30 in order to know how to smelt, it's that if you lose more than about half your population, it's going to be way easier to scrounge than to produce. There's plenty of extra lying around.)

Sufficient artifacts and knowledge from our current civilization survives, so, eventually, younger civilization can utilize it for a "reboot"

This depends wildly on what you mean by "reboot". Given enough time (say 10,000 years or so), any group of people will "reboot" society. We did it once, we can do it again. But I assume you want something faster. So let's say 100 years. What can our society do after 100 years?

Not much, it turns out. At the most optimistic, we're back into the Industrial Revolution -- maybe 1770's tech. More realistically, though, we don't even get that far.

Again, this is much less a function of age than of the sheer number of people left -- the more people, the better. Existing knowledge is almost immaterial, in some ways.

First of all, what is this society not going to be able to do? You can kiss computers goodbye, for instance. Now maybe you're about to object we built computers in less than 100 years, so surely this society, with its current knowledge, should be able to do it faster. But this is wrong.

Yes, we were able to create computers in less than a century. But we had a bunch of existing knowledge to draw on. Early computers, for instance, utilized vacuum tubes. Probably no one alive today of any age knows how to make them; almost certainly no single person knows how to mass-produce them; and even if someone did know how to mass-produce them, they wouldn't have the resources to build, run, or maintain the factory necessary to do so.

So computers are out. So are cell phones. So are regular phones, probably. How many people know how to build an old rotary phone? Even if someone did, your new society doesn't have the manpower to maintain and staff the switchboard network. When people are worried about getting enough food and water to survive the year, calling New York just doesn't seem like much of a priority.

Automobiles are similar. We discussed the issue of actually producing new goods above, but even if we ignore those difficulties, by the time people could reproduce the Model T there wouldn't be much call for a Model T. To begin with, cars run on gasoline, and your society surely doesn't have the ability to produce it. Not to mention that without any maintenance for several decades, much of your road system isn't in great shape.

Basically, if you're kicking your society back to before the Industrial Revolution, don't expect them to get back to where we are now in less than two or three centuries minimum. My guess is it will actually take them longer, because so much of the knowledge and security that people had during the Industrial Revolution is now gone. No one is working on reinventing the cotton gin when they're worried where their next meal is coming from. No one's even planting cotton.

If we lost more than half the working-age people, the resulting impact to society would be devastating. Starvation would be widespread. Society as a whole wouldn't recover for centuries, even with adults still around. With only children? Good luck with that. Depending on how many millions they lose over the first year or two, it might take them centuries just to breed their way back up to Industrial Revolution population levels.

Minimum age required to fully rebuild within 100 years: 40-50 (approximately 2/3 of the working population)

tl; dr: If you're writing Young Adult fiction, I'd say that 17-18 is your minimum age. Not because I think it would actually work, but the genre does encourage age-related suspension of belief. There have certainly been other similarly absurd scenarios that have gone on to widespread acceptance (cough Hunger Games cough).

On the other hand, if you're trying to make a serious, well-considered construction of what society would be like if all the adults suddenly disappeared...understand that probably involves massive casualty rates, especially during the first few years (but also increases in other types of mortality thereafter; expect infant mortality rates to skyrocket, for instance); a return to subsistence farming; and abandoning any technology much more complicated than the plow for anywhere from a few to several centuries.

Keeping language and basic societal structure is easy. Keeping advanced technology is nearly impossible.

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  • $\begingroup$ Thank you, Galendo. To answer your questions, maintaining tech level with scrounging is OK, and there is no time constraint on reboot - just the fact that "reboot" should be reboot using the past knowledge and not the "boot" from the scratch. $\endgroup$ – Alexander Aug 31 '18 at 20:18
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I'd say around 8 to 10 years old.

First off, any city kids will die. However, in rural area's, especially in poor countries, there's plenty kids working on their parent's farm who know enough about crop rotation and the like to run it, if they keep working. Why the poor countries ? Because in rich countries the farmers are a lot more dependent on technologies, and in poor countries they're more self-sufficient. This'll be enough to get self-sufficient. You aren't quite back to stone age - they'll know basics of both agriculture and husbandry, which will be enough. Reboots will take ages as all many of the wealthy countries, where knowledge is concentrated, will be wiped out completely. The groups of boy scouts might take a bit longer to die.

Any reboot is probably dependent on low-population-density countries that are still rich and well-educated. For example, kids in certain parts of norway learn basic survival out in the wilds, while also being taught normally in schools. These could be a focus of a reboot, if enough knowledge is preserved in books they can access.

Note that every digital dependent storage will most likely be wiped out completely.

You'll probably also have traumatized every single one of them. But humanity is tough, and children can be better at adaptation than you'd think. After a year or five, it's just the way life is (and they'll start marrying again). They'll almost have forgotten the world used to be different. Most likely, older siblings will try and take care of their younger siblings, so those probably won't all die, but the lower you get in age the higher the mortality goes.

Right away, civilization will fall down close to stone age levels. Groups will either go agricultural, hunter-gatherer or animal-herding. Or some combination of those. They'll lose global communication as soon as automatic systems start failing. Electricity will fall out, and that's that. From that moment on, every group is on itself. Oh, yeah. They'll form groups, or tribes, before the week is over. Most of the ones fated to die as well.

In a year of five, the survivors will have a more or less stable life. This means they might start attempting to harvest technology. Expect the first new children to be born around this time - about 12 is a pretty normal age to be fertile, and they'll start it right away. Perhaps there are exceptions before, but as a rule humans are conditioned to start families by choice when they feel more or less stable and capable of feeding the offspring. This goes for most animals. 5 Years after the collapse sounds good to feel stable again. At this moment, basically everyone will have only distant memories of the previous world, or none at all.

Note that there will be all weird kinds of religion or random beliefs popping up left, right and center. Humans look for structure in the world. Also, there are a lot of books kids could read, but at this time, the current world might be a dream for all most know. They won't be able to tell the difference anymore between a lot of science or science fiction - someone who finds a star trek book might consider it historical, and therefore conclude that they are are plenty other worlds containing humans out there. Or people read harry potter and attempt magic. At this point, high school educational material should be easy to find, and this may help them.

After about 20 years, you're probably in medieval level society again, at least the more developed parts of the world. From there up, it depends on how smart the ones are that try and harvest knowledge left behind, and how accessible that is.

College level educational books will still be common around this period, and most science up to about 50 years before collapse will probably be preserved in one form or another inside those.

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IMO the answer is "older than you might think".

The glue that holds civilization together is communication. A functioning wide-area network using modern electronic communications is soon going to break down, as key devices fail and don't get fixed.

So, who in your post-disaster world actually knows how to run a society where most of the comminucation network consists of telephones connected by physical wires, or the physical delivery of messages written with pen and ink on paper? Arguably, only people who were alive when that was the only thing available to them - and that probably means "people over 50", at the youngest.

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    $\begingroup$ Thank you, but why the loss of communication should kill autonomous communities? $\endgroup$ – Alexander Aug 31 '18 at 20:11
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A bare minimum scenario:

  • There are a few isolated islands or valleys,
  • Each of which has some children of ages four - six who know how to read.
  • These islands or valleys happen to have very hospitable ecosystems.
  • That produce year-round food supplies that do not need cooking.
  • The islands or valleys are not subject to fatal heat waves or cold spells.
  • A large fraction of the books available teach practical skills for rebuilding civilization.

For best results, the young children do not have to care for any babies, toddlers, or other children younger than themselves.

In this scenario, the entire world does revert to the Stone Age. Most of the world's D-Day+1 population dies within a year. But starting about ten years after the disaster, the literate children in the surviving pockets become strong enough to build new homes, start signal fires, make new boats, and find each other.

Lord of the Flies and Orson Scott Card's Ender's Shadow present sadly plausible scenarios for the social dysfunctions such societies might have. Jane Auel's Clan of the Cave Bear novels and Scott O'Dell's Island of the Blue Dolphins have more hopeful scenarios.

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  • $\begingroup$ 'above stone age' as requested means you need at least basic metal working like smelting, casting and forging. $\endgroup$ – Burki Sep 3 '18 at 12:44
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At any age less than ~40 collapse is certain, modern civilization just relies on too much specialized labor, you can't remove a large part of of population and not have collapse. The older the cut off is the less persistent technology will be lost, but no matter what you are looking a a reset to preindustrial/WW1 tech levels depending on what specific tech you are talking about. Oddly the less developed countries will be the ones that are the best off since said children will have actual work experience and will have apprentice like understanding of more advanced skills and will have a large number of farmers.

Below teenage a small percentage will survive but you are talking about a reset to stone age or close too it. Some children will possess some specialized skills but they will be few and too far separated to really take advantage of it, and there will be a huge die off afterwards caused by mass starvation. There will be no rebooting such a collapse they will have to progress normally as if modern civilization never existed.

The real questions is what technologies do you want to loose and which do you want to keep. For instance keeping some of modern medicine will require something in mid to late 30's. It's not enough that a few some people with skill X survive it is you need many many people with the skill to survive the initial die off becasue only some of them will survive all the subsequent die offs collapse will trigger. the more severe the collapse the more tech will be lost not becasue no one has the knowledge but because people are too busy trying to survive to preserve the knowledge.

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  • $\begingroup$ Thank you, John, we just need to keep it above the stone age. Modern medicine is not required for that, however, infectious diseases will be an issue, and even more so if basic medical knowledge is lost. $\endgroup$ – Alexander Aug 31 '18 at 16:43
  • $\begingroup$ modern medicine is just an example, above stone age is really vague, do you want iron working, glass making, agriculture, literacy? If you want metalworking for instance you are probably looking at something in 30's as well, mining and smelting are just rare knowledge these days. remember easily accessible metal deposits are all but gone these days as well. $\endgroup$ – John Aug 31 '18 at 16:51
  • $\begingroup$ Yes, I want metalworking, and at least one person in the world who still can read and write. Everything else is nice to have, but technically not required. For metalworking, there is one big caveat - it does need to be a full technology cycle. People can use scrap metal, for as long as it's available. Also FYI iron ores are still quite widespread, but for many other metals mining is indeed difficult. $\endgroup$ – Alexander Aug 31 '18 at 17:18
  • $\begingroup$ you should lay out these requirements more clearly in your question. Many will not see these comments. $\endgroup$ – John Aug 31 '18 at 20:51
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For a given culture, the cutoff would be the age that kids generally accept responsibility for others.

As you lower the age, the death rate increases.

As the current tech level increases, the death rate increases.

So urban north american city with it's narcissistic youth you would get massive die offs even with twenty somethings. That tech is hard to maintain without the people who ran it, and too much isn't written in books.

Small town rural north america with numbers of farm kids, and some old simple equipment (a 40 year old diesel tractor is a simple machine) probably would do better than the city even with a cutoff of 15.

Far east rice paddy culture? Big enough to harness and control the water buffalo.

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The way things are going, higher than whatever it is right now...

How low this X can be that human civilization survives?

Given you mean human civilization above stone age, and not necessarily this one, nor any particular civilization.

  • language surprisingly, humans seem to spontaneously develop language, without any help at all, as the Nicaraguan sign language suggests. This fits with Noam Chomsky and the "language instinct" of Pinker and others. It might not be English or related to any present language... but assuming text (advertising, signs, books, perhaps computers etc), it's gonna be influenced by existing languages.

  • tech (asuming machines still exist, and are in working order) just using much of todays technology is not that hard. Consider also that not everyone has to be able to do, but just that there are some who can work it out. Of course, they won't use well (first anyway),. and there'll be mistakes, some serious, some catastrophic (locally).

Even most repairs could be it will be managed more or less.

The problems will come from serious repairs, and chains of dependency. e.g. iron foundaries, power stations, tool making tools, silicon lithography.

And then, being able to improve on them, which requires theory and excellent prototyping skills and tools.

  • knowledge arguably, the renaissance was a reboot after the dark ages (at least in europe), using classical knowledge which was preserved. That took a while. I do think today's texbooks and online resources (assuming they kept working for a while) would be enough for some children to re-educate themselves. Though, there are oral traditions and attitudes even within fields as "pure" as maths, which wouldn't be properly passed on, and would have to be rediscovered or redeveloped (perhaps somewhat differently).

Given all this, I think the age could be extremely young, such as 5 years old or even 2 or 3... perhaps 6 months The barrier is whether the children could survive without carers - perhaps even younger, provided some survive. Arguably, the human infant is born prematurely, and continues gestation ex-uterus. Birds are similar, with constructed nests. But this is an immediate, short-term consideration, and not what you're interested in.

The next problem is having enough food and shelter etc for all this learning to occur. But if we assume a stocks of food (NB: fewer people makes stock last longer, and the mechanisms of production still exist, this is not too hard.

but...

However, I think the big problem is the survival of a culture that avoids war and conflict enough for all this to occur. People - including children - naturally form groups (gangs). In the world right now, there are several places that would not be able to do this. Historically, war and conflict is common, and regions and times with stabliity sufficient for development is rare.

Related to this is the institutions that make it easier to avoid conflict than escallate, from councils, police, up to disputes between immensely powerful corporations and even governments that meekly accept judicial decisions, and a judiciary that, by obsession on technical correctness, is more-or-less not corrupt.

So, getting along is the problem... But I guess that's what you mean by "civilization". I'm not sure what age would enable this to survive...

I'm not even sure we can do it today, with whatever it is right now.

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