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Recently, I asked How would government change if everyone died by the age of 25? as something of a thought experiment. After some consideration, though I've developed the premise a bit further, and I'm working on more aspects of the world.

Some of the basic points (including things that I've changed):

  • The land of Xenqu is, to the knowledge of its inhabitants, the only inhabited place on the world, though they haven't explored much of it yet.
  • They're at a medieval level of technology.
  • They've developed a hereditary disease, which began in the year [X]. At that point, anybody who was 25 or older died instantly. From then on, anybody who was descended from some Xenquan dies on their 25th birthday.

Is this possible? Can some sort of biological weapon (so it doesn't have to be an actual disease) exist that is applied once (in the year [X]) and thereafter kills all 25-year-old Xenquans? I'll fall back on magic if I really have to - I'm envisioning a case where a dying evil master magician casts a curse on the land - but that's only as a last resort.

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    $\begingroup$ I think the adage is particularly apt in this case: "Age ain't nothin' but a number." $\endgroup$ – Caleb Hines Nov 17 '15 at 2:58
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    $\begingroup$ This was a plot point in the classic RTS Dark Reign. One of the factions had a DNA marker that killed them on their 25th birthday. tvtropes.org/pmwiki/pmwiki.php/VideoGame/DarkReign $\endgroup$ – Travis Christian Apr 21 '16 at 20:20
  • $\begingroup$ Come back and pick an answer? Please? Or a bounty? Pretty please? :D $\endgroup$ – AndreiROM Apr 21 '16 at 21:48
  • $\begingroup$ Keep in mind that your population won't survive a bit more than a few generations. Living just 25 years while trying to raise human children means that on the best cases you'll live with your kids for just 10 years - you won't have enough time to teach and keep knowledge alive. $\endgroup$ – T. Sar May 24 '16 at 14:03
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    $\begingroup$ When I read the title "How can I kill everyone by the age of 25?" I thought it would be about how you could kill everyone before you turned age 25. $\endgroup$ – Evan Rosica Jun 2 '17 at 9:01
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Your entire world is an immersive virtual reality game, played by future humans whose technology has cured death. They are immortal and bored, so most of the time, they play video games about the good-old day when life was short.

...and 25 years is the amount of time you get for a quarter.

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    $\begingroup$ Reminds me of a recent Rick and Morty episode... $\endgroup$ – Samuel Nov 18 '15 at 22:49
  • $\begingroup$ Thanks! I wasn't familiar with the series. Will check it out! $\endgroup$ – Henry Taylor Nov 19 '15 at 6:15
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    $\begingroup$ +1 for pure hilarity $\endgroup$ – Quiquȅ Nov 22 '15 at 1:57
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The problem with something like a disease, or radiation, etc is that the "disease" has no way of knowing when you turn 25. It's also highly suspicious that it doesn't seem to affect you at all growing up, but then suddenly you die on your exact birthday. It's ridiculous.

More realistically, this disease would kill you around your 25th birthday - with some very strong individuals living to be quite old (27 say), and some weaklings dying at 20. Then it becomes more realistic.

1) Nanobots that infiltrated the population from the wreck of a crashed alien drone (in their medieval views their people scavenged sky metal and suffered a curse). These nanobots infected the hosts, etc.

2) A curse on their ancestors cast by a God/powerful wizard. (oops, you covered that)

3) That's just their genetic make-up. When you really get down to it, you don't really have to explain it. None of the characters would know what "genetics" are anyway, so how could they "talk" about it and explain it to the reader?

4) The radiation of their star slowly saturates your body and kills you around the age of 25 - because that's just how the timing works out (this premise if kinda shaky for some very obvious reasons, but hey, it's your imaginary universe)

5) On their world all people are infected with a parasite that is ubiquitous - simply can't be avoided - everyone is infected in the womb, or shortly thereafter. When that parasite "matures", or it itself dies, it's death/departure kills the host.

This parasite/human relationship is symbiotic, and the human receives some benefit from it. When the parasite leaves the host, its death is somehow beneficial to the world as a whole, and thus people see their short lives as being lived in sacrifice to their world.

Freaky as hell, and a pretty good explanation as such things go.

I can adjust some scenarios and flesh them out on request.


I'm going to roll with the parasite, and make this stuff right up:

Scenarios:

a) The world was a perfectly normal place, as this universe goes. Then one day, an alien craft/asteroid crashes on the planet. This craft contains a parasitic life-form which attaches itself to its host, and spreads aggressively: perhaps it releases spores into the water/air which infect the host and cause that malignant life-form to grow inside its victim. There is simply no way to remove it without killing the host. Being of alien origin, this life-form has a life-span of its own: exactly 25 years. The victim dies not on their own 25th birthday, but on the 25th "birthday" of their parasite, which may have infected them a few days after birth, for example.

b) The world was a perfectly normal place, until one day an alien craft/asteroid crashes on the planet. This craft contains an alien life-form which is highly intelligent, but parasitic in nature, and which cannot survive for long on this world because of certain factors (radiation from the Sun, etc.).

This life-form uses psychic powers to communicate with the human elders. They propose a trade, to be accepted by the elders on behalf of all their descendants, or until a "rescue craft" arrives to take the aliens away (which might take many centuries to get here, but the aliens are crafty, and don't say this outright):

To allow their species to symbiotically attach to human hosts, in return for great power to those who "link" with them. However, there was a catch in the deal that the elders, foolishly thinking they could trick the aliens, accepted. When the host reaches the age of 25 - physical maturity, and thus greatest vitality - the alien claims its prize for having enhanced the human for all that time: it completely drains them of vitality, killing them, but extending the life of the alien, which is then passed to another host.

There can be great benefit to the alien attaching itself to a human: they might reach maturity faster, develop greater strength and intellect, as well as being immune to diseases, healing injuries faster, etc.

Any who try to detach the alien, are killed by the alien host. Any who try to fight the aliens are subdued via psychic powers. After all, a deal is a deal.

Over time, it can work out that a great plague wiped out all the humans that were not symbiotically attached to an alien, however the "survivors" are doomed as well, because their means of surviving the disease kills them upon reaching their 25th birthday.

-- is there any interest in me expanding on this idea, or am I beating a dead horse?

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    $\begingroup$ It's a nice list of possibilities, but not really an answer. Perhaps you can pick one and explain how that option would A) kill everyone over the age of 25 when it started and B) kill everyone else on their 25th birthday. It looks like one of your five options might fit those criteria. $\endgroup$ – Samuel Nov 16 '15 at 22:58
  • $\begingroup$ Regarding 3: Why wouldn't it have happened before, then? I'm also kind of looking for a more in-depth explanation than that - same goes for 5. $\endgroup$ – HDE 226868 Nov 16 '15 at 22:58
  • $\begingroup$ @HDE - My fav scenario is number 5, tbh. It has an Alien vibe to it that I absolutely love. Those parasites might have come to their world on a crashed ship, or asteroid (originated off-planet). Or, going back to magic, some unfortunate wizard created them as a "bionic implant" to boost certain human features, such as intelligence, etc, but his amazing invention has the unfortunate side-effect of being lethal when the "implant" reaches a certain age - 25 years. I'll edit and add more details $\endgroup$ – AndreiROM Nov 16 '15 at 23:02
  • $\begingroup$ As for diseases, HIV/AIDS is quite slow to develop. Make it even slower and more virulent, and give it time to spread and there you are. Prions may also be quite slow. I wonder if prions can spread via milk... $\endgroup$ – Dallaylaen Nov 17 '15 at 0:18
  • $\begingroup$ The problem with option five is it simply pushes the problem to another organism aging to be exactly 25 years old. Besides, an alien wouldn't have the same length of year, so it seems to strain plausibility that it happens to line up so well. $\endgroup$ – Samuel Nov 18 '15 at 22:49
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I like the nanobot idea, but I'm not sold on the alien source. Let's toss out the aliens and look for a purely human motive to have something happen at 25. For example, a brilliant nanotech engineer sees the movie In Time and agrees with one of the ideas from it: people reach a physical and mental peak of maturity at 25. It's all down hill from there. So the plan is to freeze aging at 25.

The first part of this is successful. Frankenstein (all the best reckless inventors are named Frankenstein) develops a method that allows nanobots to determine the age of any human with great accuracy.

The next part doesn't go as well. The idea was to stop aging by repairing the naturally occurring damage to the cells and grow new ones to replace the most damaged. Unfortunately, rather than making just one replacement, it keeps on making replacements. This has much the effect that cancer does, plus it uses up all the body's nutrients. People can't digest their food fast enough to stay alive.

The same section of code that was supposed to limit the cell replacement was intended to limit production of new nanobots. So the error existed in both and instead of a limited number of nanobots for experimentation, they reproduce as many times as they can. Only one made it out of the lab, but that was all it took.

It triggers in everyone 25 or older and lies dormant in those who are younger. Since the young seem unharmed, they don't realize that they need to quarantine them until it is too late. Also, it is difficult to quarantine. They're nanobots, not bacteria. If they don't get proper nutrition/fuel, the nanobots hibernate until they get some. Everyone working on the experiments is 25 or older, so they all die.

This gives you something like you want. There's a single event that kills everyone over 25. If you're willing to believe that the nanobots can accurately measure age that close, you can even get people dying exactly on their birthdays going forward. It's not quite instantaneous, but it is relatively quick. If instantaneous is necessary, you may be able to come up with a different problem with the process.

Note that it's quite possible that the nanobots are more accurate if they've been with someone since birth. Then they just need to measure time since the placenta is disconnected. As a backup, they can still use whatever age measuring method to confirm. So the initial event may kill everyone older than 34, most people 25-34, some people 16-25, and no one younger than 16 (blood tests for age have an error up to 9 years; examination of teeth and bones may allow for additional accuracy). Once everyone who was alive at the time of the event dies (within 25 years), the more accurate method would be usable with everyone.

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Xenqua isn't on Earth is it? These are aliens. So ... Xenquans expect to die in their 77th year. A few reach 90. A few die young at 64. Spot the pattern. Of course it is not universal. There are accidental deaths at any age.

Here on Earth we have organisms that live precisely 13 years. Never 12 or 14. Related creatures live 7 or 11 or 17 years. They are cicadas.

Xenquans have a built in 13 year timer started at conception. It triggers puberty at 12, which is rather more drastic for them than us. One very important thing that puberty does is to cause maturity of a gland which then secretes an inhibitory hormone. So the next time their 13 year timer ticks, they do not re-enter puberty. But later in life that gland weakens and their bodies go into total organ failure caused by the same mechanism that produces puberty the first time around.

And now there is a plague which destroys this gland. Puberty at 12 years. Death at 25 years. Easy wasn't it?

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Make the trait that kills by age 25 also convey greater fitness until death

How is this going to spread in the population? If this genetic disease makes a carrier less fit than competing individuals then the disease will die out. However, if the disease dramatically increases fitness compared to non-carriers then in a few generations the entire population will have this trait/disease. It will be up to the author to decide what trait provides greater fitness: perhaps greater strength, greater resistance to poisons, greater intellect, brighter/flashier feathers (sexual selection)...the list is endless.

The rest of the answer will assume human-like development and reproductive capabilities.

In year [X], someone got this mutation either at conception or later by a virus...just like humans get genetic diseases on Earth. Personally, the virus approach seems to work best because a virus can spread through a population fairly quickly (days or weeks) while mutations at conception only effect a single person.

There's any number of ways to make this mutation kill the carrier at or by age 25.

  • Maybe there's a metabolic by-product that accumulates to lethal levels by age 25. Maple Syrup Urine Disease is an excellent example of this in humans.
  • An auto-immune disorder that ramps up approaching age 25.
  • Mental health disorder, say deep depression that leads to suicide.

As long as the mutation conveys greater fitness but at the cost of decreased life span, it should satisfy the requirements stated.

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According to the 1970 hippie-fantasy movie Gas-S-S, a human's neurons start dying off at age 25. A chemist came up with a gas that made that accelerated the depletion; so that a person ages normally, but when he reaches 25, he dies from the depletion in a short time.

Would something like that work? The "hereditary disease" causes rapid neuron depletion, but only in people who would have that depletion normally?

Also, see the episode "Miri" from the original Star Trek.

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Here's some graphs:

enter image description here enter image description here enter image description here

Also think about their derivatives. Basically, when these things hit a certain level, the person dies. For example, maybe a parasite grows larger when more Testosterone is produced; when Testosterone starts declining, the parasite freaks out and kills the host. Similar explanations are possible.

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There is a tropical disease called "Mal de Chagas", or Chagas Disease, caused by a parasite called trypanosoma cruzi. After an acute phase, with a very high survival rate, 20~40% of the infected get into the "chronic" phase, on which you start developing life-threatening heart and/or digestive disorders And die several years (10~20) latter, mostly form a sudden heart failure. A very non sciencey short explanations works like this: The parasites looks for the internal organs (principally the heart, and the digestive track) and just kinda lay there. Then the human's immune system goes and kills the parasites, but they also damage nearby tissue in the process. This tissue damage is what actually kills the infected person. The Chagas disease is transmitted orally (by eating food or drinking water where an infected bug defecated) or by blood.

You could work with a similar idea. A parasite, that's found on their water/food source, and kills 25ish years latter. They contract the disease at early age, their immune system kills the parasite (or on a few cases they just die a couple of week after being born form the infection, but the Xenquans have yet to make the relation with their 25 years cap, and think that is completely normal having fever after being born) but their heart suffers some damage. They live a relatively normal life, but with some symptoms as irregular heart beats form time to time. Eventually, between 20 and 30 years latter they simply suffer a heart attack.

Pros:

  • Its a non magic solution to kill everyone at ~25 years old
  • The sudden heart attack goes nicely with your initial idea of suddenly dying, and any rudimentary autopsy will simple result in "heart failure"

Cons:

  • They would not simply die at 25. They would die somewhere between 20 and 30, sometimes even earlier, depending on the particular heart strength. But due natural selection, those of weak heart would not live enough to have children, so the average will be 20+
  • They would also suffer form some symptoms, as irregular heartbeats form time time, and those symptoms would progressively worsen.

I can't hardly think of a natural way that kills exactly at 25 years, and even human engineered ones also seems improbable. My example of Chaga's disease was the closest I could find

EDIT: Since I don't know the effects of Chaga's disease on newborns, and I highly doubt that they are the same of an adult's symptom. You could make the parasite to be found on a particular water/food source, and that the Xenquans have an adulthood rite (which would be half of their life expectancy) involving said contaminated source. OR maybe they have something like beer, which only "adults" can drink, and thus they contract the infection at 12~13 and still get the ~25years kill (just make the parasite a bit more aggressive).

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  • $\begingroup$ Btw Im not a native english speaker, so excuse any grammar or redaction mistakes $\endgroup$ – Silver Nov 17 '15 at 18:28
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The mutation rate is abnormally high (at least compared to our own world). By the time a person is 25, they have accumulated enough 'noise' in their DNA that their organs don't continue to grow themselves according to the proper pattern.
It could be caused by mutagens in the environment, or their planet's sun could generate more EM radiation than ours does.

Pros:
Not magic, and it could easily happen.

Cons:
It would not kill the person exactly at 25. Though, as AndraiROM noted:

The problem with something like a disease, or radiation, etc is that the "disease" has no way of knowing when you turn 25. It's also highly suspicious that it doesn't seem to affect you at all growing up, but then suddenly you die on your exact birthday. It's ridiculous.

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