Aliens from the planet Zog have activated a ray which will surround the entire earth and make all human beings the same age. (Let's not worry about why: maybe they want to cripple humanity - in which case, replace Zog with Skaro - or maybe they think they're doing us a favour by unifying us.)

They just need to enter a fixed age $X$ into the control panel, and then the ray will take effect. Everyone younger than $X$ will find their bodies ageing instantaneously, while their minds stay the same; everyone older than $X$ will regress to exactly the same state - in body, mind, and experience - as they were when they were aged $X$.

The effects are permanent; the ray envelops the earth forever and continues to affect everyone, kicking in roughly once a day (think of it like updates automatically running on your computer). The human race effectively becomes immortal, their bodies staying at age $X$ for ever. Babies' bodies age to those of $X$-year-old people within the first day of their lives. Anyone who was older than $X$ before the ray has their short-term memory completely screwed, forgetting each day once it's happened, and the same happens to the rest once enough time has passed that they would be aged $X$ even if not for the ray. (In other words, nobody can ever have more than ($X$ plus one day)'s worth of experience.) People can still die in all the same ways they could before, except old age.

The aliens allow us to choose the value of $X$. It can be any age from that of the youngest person in the world to the oldest.

  • What value of $X$ would be best for humanity, in the long run? Obviously there are pros and cons to either a larger or a smaller value of $X$, but make the best case you can for a ball-park figure for the optimal $X$.

  • What will be the effects on society of the change? How will it affect interpersonal relationships, bearing children, the increase of human knowledge, etc.?

(Please check the tags - I don't have much experience on this site, and am not sure if they're appropriate for this question.)

  • $\begingroup$ If the people that aged-down can't remember anything past age 'X', they can't do anything, because they can't get any new information, even for a second! ... They'd be mentally frozen at the time of their 'X' birthday... They can't answer anyone or do anything, because that requires absorption of new information.... Unless you mean "New information rewrites old information, leaving the total amount of experience = 'X' Years" ... If so, I hope you can choose what to keep, else you might forget who you are, or potty training or something else important... $\endgroup$
    – Malady
    Jun 21, 2015 at 20:37
  • $\begingroup$ @Malandy Think of it as like Alzheimer's disease maybe? Your idea of "new information rewrites old information" is interesting, though not what I was thinking of. It'd be hard to make that seem realistic whether or not you can choose what to keep! How do you measure 'amounts' of information/experience, for instance? $\endgroup$ Jun 21, 2015 at 20:46
  • $\begingroup$ Another idea is to make the effects of the ray kick in just once per day, so you get up to X years plus one day of experience. Then it's just a case of "you reach a certain age and suddenly your short-term memory is screwed", which makes it even more like Alzheimer's. Would it be OK for me to edit the question to make this (slight) change? $\endgroup$ Jun 21, 2015 at 20:47
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    $\begingroup$ If your question is definitely interesting, it is however hard to decide objectively on which is the best solution. You could potentially have several answers, say 15, 30, 40, etc. each with their merits. How are you going to decide which is best? It would be appropriate for the side to add an objective critera to help deciding. That will also help the answerers to know what you are looking for. Please have a look at the too broad section of the help. $\endgroup$ Jun 22, 2015 at 8:19
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    $\begingroup$ Do pregnant women explode? $\endgroup$ Jun 22, 2015 at 14:46

5 Answers 5


I'd like to decompose the alien intervention into two very distinct factors:

  • mental and physical rollback every day after age X;
  • children and young people look like adults but in other regards are perfectly normal.

rollback as viewed by a person

The easiest part of the situation to assess is how you feel when you are over X. Basically it's like the day after the X'th birthday is the last day of your life only it exists in numerous variations. The yesterday- and tomorrow-you are more like your clones (in scifi not biological sense) than a direct continuation of your personality. So it's natural to make the most of these 24 hours or at least to try to come to peace with yourself.

The first X-generation has an additional challenge in grasping the situation since for them it would look like a completely unanticipated time-leap into the future (think of their young personalities) where they have only one day to spend until the rewind. Even in later X-generations the time-travel aspect is still there only you know why it happens. Anyway you still have to learn a lot again and again.

It seems like those over X (let's call them struldbrugs for brevity) would converge to the same daily routine after some time. Imagine you are a struldbrug and you come up with a list of things to do during this day. The list is likely to be the same each time because you start with the same state of mind. Then you realise that some things have already been done during the timeskip by other "yourselves" (and after some time only repetitive tasks are going stay on the list). Then you receive a memo from the yesterday-you saying that this thing on your list is not actually worth it and you can't really pull off that thing due to your condition and then it gives you hints on how you properly do other things. So in the end you stay with simple, pleasurable, repetitive and highly optimised activities. Also not very expensive since you are burning through your savings over time. You may ask why some of this doesn't apply to people in our world. The main differences are:

  • people change, learn and acquire new interests — struldbrugs don't

  • struldbrugs have hard time making plans that span over more than one day

  • struldbrugs never get bored: they may have the same conversation or visit the same place over and over and their impressions are always fresh

rollback as viewed by society

A direct consequence of the above is that struldbrugs tend to be very disoriented and/or self-absorbed and are not going to be economically active. At some point in time they become completely dependent on the goodwill and care of people around them. Which causes an economic and ecological collapse: they don't work and don't die.

Still we can find a couple of ways around this problem:

  • Deceive struldbrugs into thinking there'll be the next day, put them with other struldbrugs into a stable environment and assign a repetitive task. They won't even notice the time passes, never get tired or ask for payment. The downside is: slavery is unethical and economically inefficient. And there is no way you can put them through training to acquire new skills.

  • [this one is probably exploiting a loophole in the question] What exactly is encompassed by the physical rollback to age X? Obviously shortening of the telomeres and random mutations caused by background radiation must be reversed. Probably teeth decay. Does liver decay count as aging? Do their diseases progress over time? Do they have to eat and sleep or their hunger and drowsiness disappear after midnight? Some of this may grant struldbrugs superhuman abilities that make them more useful to the society and they may survive on really inexpensive diet if malnourishment is not an issue.

  • Admittedly the two previous suggestions only slightly alleviate the difficulties even if struldbrugs are effectively undead. Humanity would have to expand fast to currently uninhabited places and ultimately — to other planets to accommodate the undying population.

  • The more realistic solution is though just to euthanize struldbrugs after they reach a certain age. Give them 10 or 20 years to complete their perfect daily routine and then let them rest in peace.

instant adulthood and conclusion

Instant adulthood involves some radical changes on physiological level across the population but doesn't have nearly as profound and catastrophic consequences as the daily rollback.

Given that the question of what X do you choose essentially means "at what age are you ready to end your productive life?", the answer should be close to the age of retirement, but considering young people being given the same kind of body we must care about reproduction. So the final answer is about 35-40 years — the upper bound of risk-free child-bearing age for an average woman.



It's post-puberty, you get nearly all the knowledge of the post-college people, and the body hasn't started deteriorating yet.

It also means that babies get 27-year-old brains and bodies immediately when they're born... But, they can learn for 27 years...

Eventually, people will figure out how to teach 27-year-old babies how to function in their new society...

So, then, assuming that knowledge takes... 1-2 years to learn...

They have 25 years to learn as much as they can...

Their whole society is centered on learning. Well, that and the basics of human survival...

People older than 27, and already in a sexual/romantic relationship would most likely enjoy the new, younger bodies...

The younger ones... A 27-year-old's body is very different from a 10-year-old's...

So, people know aliens exist... Religions start changing...

Politicians get aged-down... politics collapses?

Prisons may not work, due to immortality and inability to learn...

People who were injured on their 27th birthdays, or were in withdrawal from smoking or whatever, are either gonna commit suicide, deal with it, or try and find a way to change their "default" bodies so they don't suck.

Tattoos may become more popular if the body is reset every once in a while, so they're less permanent?

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    $\begingroup$ Why not 25, 26, 28, 29, or 30? $\endgroup$
    – HDE 226868
    Jun 21, 2015 at 20:33
  • $\begingroup$ @HDE226868 - Hmm... Gonna range it to 25-35... Do you think 25-year-old prodigy doctors know everything they'd need to know in their specialty, for day-to-day doctoring? $\endgroup$
    – Malady
    Jun 21, 2015 at 20:40
  • $\begingroup$ The babies and small children would all need to be detained/enclosed to stop them killing people when they have a tantrum. There's a reason that we don't grow as fast as other species, and it's that our physical development (much slower than it could be, eg a horse grows to its adult weight of eg 400-500 kilos in five years, compared to a human growing to eg 100 kilos in 20 years) needs to stay in sync with our mental development (which proceeds optimally as far as we know). $\endgroup$ Jun 23, 2015 at 15:04
  • $\begingroup$ @MaxWilliams - Well, that's the "Figure out how to teach babies" thing, droo46's answer goes into more detail on that... $\endgroup$
    – Malady
    Jun 23, 2015 at 15:39

Regardless of which age you choose, one of the big issues you'll run into right off is having young minds in old bodies. The Worthing Saga has a small section in it where adult bodies in cryo lose their memories and basically act like infants. The one person there who hasn't lost his memory has to do everything you would for a baby, but it's complicated by the fact that the baby is just as big and strong as their caretaker. They progress quickly, but are very dangerous at first.

If humanity could choose the ideal age, I agree with the other answers that it would fall on the lower end of 25-35. Most people's physical peak was in their twenties. This brings up another question though: If we're choosing our perfect age, how does that affect health? You mention immortality, but are diseases and maladies cured?

In a world where aging is gone, childbirth is no longer possible unless you've got some sort of loophole to allow for fetuses to age to adult bodies the day after they're born. If that's the case and no one dies from natural causes, I would imagine that people would be very careful about bearing children, both because of population control and because of the added dangers in raising babies in adult bodies that I mentioned above.

In this world, age becomes a less important metric. I would bet everyone would still keep track for the most part, but life milestones would be based more on maturity. School grades are grouped by mental acuity and achievement instead of age. Some people "grow up" quickly and move through school and out on their own as soon as they are able, and some people rely on their parents for a longer period of time. These become personal decisions rather than eventualities determined by time. There will people all along a spectrum of pushing life to the fullest, and those who hang on to childish things.

EDIT: Just noticed the memory thing. That changes things a bit. If that's the case, the years you have before your memory is frozen are extremely valuable because you have to do all of your life before that. Of course, jobs that require that the same thing be done every day would be great for those who hit the limit. The first thing that comes to mind is milking cows; you would just have to have someone younger manage the herd, but the daily tasks could be done by a "Freezie".


I was going to say 35-40 because people have enough experience in their lives to not make as many dumb mistakes, and the bodies are still in good working order.
But when you hit that magic number, you enter ground hog day, in reverse, every day you get farther and farther from the last day you remember anything. So then I was thinking 85, you will remember most of your life and many start forgetting things by then any way!

But since this is ultimately going to kill off humanity, since it would be an end to pregnancy (and thus accidents would slowly kill off the rest of us), that we should all enjoy our remaining days as 16-20 yo. It would likely kill us off faster but when you can't remember the past what do you care about the future?

Mothers would either abort, or be pregnant forever, or their fetus would instantly age to an adult killing one or both of them...


I think best option is to choose no number. If you enter 35 you know that when you will have 35 years + 1 day you are going to die. Anything happened between 35 years and 35 years +1 day is going to get erased, so basically you stop living at 35.

I think those aliens are dumb, because their ray assume human life is an equation where you have two terms and you can maximize somehow the result by inputting the right number.

People with adult bodies and children mind will probably be much more less rational, there will be probably more raping because certain impulses would just happen without a rational mind controlling them and people would have strong enough bodies.

Also people would probably just not spend time learning, because they know once they reach 35 they will anyway "heal" back every day, so there will not be any need for most medicines.

You'll grow already knowing you are going to die at a fixed age, and even worse after that day you'll know that every day is unique and that your are going to die within 1 day.

Let a person have such age, on his last day he would look at calendar, assume that year was 2015. Tomorrow he just wakeup he see the calendar "year 452345", and think "ok let's live again my last day for the (450330 *365)th time, this evening I'll die".

I think families would just get trapped into a loop, if you have a family at age 35 and you know you have only 1 day to live, you will spend the whole day with your family, so you are not going to do anything else, and hence nobody could just force you "spend your life learning" because on your "last eternal day" you don't want to work using your knowledge, you want just to stay with people you love.

How do humanity could reach an agreement to such age? If at any time we have 70% of population with more than X years that is just asking to 70% of the population to die.

It also depends on wich definition of "immortal" you use. If immortal is "being able to live N years, and accumulate N years worth of experience, where N is limitless" then those aliens are not in anyway making humanity immortal.


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