Here's one to ponder: what would an immortal eight-year-old be like after one thousand years?

In most stories involving immortality, the immortals are fully formed adults. When those stories do involve children, often those children are effectively presented as fully formed adults with the body of a child, or they are presented as being "frozen in time," that is, even after a thousand years they still act, think, and behave like a regular child.

But a thousand years is a long time. They'd inevitably see some crazy stuff and have to work through it, sometimes alone. And the brain of an eight-year-old is physically incapable of certain reasoning.

So: how would the wisdom of a thousand years manifest through such a physically underdeveloped mind?

If this concept has been explored anywhere, I haven't seen it and please let me know. I can't quite figure out the right Google phrasing to find anything worthwhile.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Aug 14, 2018 at 5:43
  • 1
    $\begingroup$ This isn't a complete answer, so I'm including it as a comment. Harlan Ellison explores this to a slight extent in the story "Jeffty is Five," which was included in his Shatterday collection. $\endgroup$
    – Deacon
    Aug 14, 2018 at 12:58
  • $\begingroup$ Reading some of the answers, perhaps you should clarify - are you asking about a normal neurotypical child, or is the discussion open to outliers, child prodigies, children who already have extreme exceptionalities? A child who had, say, exceptional music ability might respond differently than a child with, say, exceptional chess ability (a thousand years of chess experience?) High-functioning autistics with advanced abilities in one cognitive area would perhaps not respond emotionally the same way that a child who had particularly well-developed social skill structures in the brain. $\endgroup$ Aug 14, 2018 at 16:54
  • $\begingroup$ And why did I just have this vision of an incurably clinically suicidal pre-pubescent immortal child going through the childhood angst of puberty for a thousand years? $\endgroup$ Aug 14, 2018 at 17:26

12 Answers 12


Have a look at real-world instances of children who appear biologically frozen in time and parallel their behavior as an indicator of how your immortal child might act.

Examples include:

Be aware that these real-life examples are most likely attributed to some form of brain damage so subjective comparisons would have to be made to the behavior of normal children of that biological age as well.

An underdeveloped mind may be incapable of understanding concepts which come only through achieving certain biological and psychological milestones as a result of the brain continuing to mature after birth.

Also note that children of a certain biological age may be incapable of understanding certain concepts due to an underdeveloped brain. While an underdeveloped mind may be able to amass knowledge, they may forever be incapable fully realizing certain concepts which mark "milestones" of development.

I find it but there is a video somewhere that shows how a child of a certain age can't understand that a puppet who doesn't observe something has no knowledge of that thing. For example, the adult shows a puppet to the child and a ball. The puppet is put into a box and the ball is placed into a colored bag. Then puppet is removed from the box and the child is asked if the puppet knows which bag the ball is in. The child always thinks that the puppet knows because the child saw where it went. Or something to this effect. I would love it if someone could find this video for reference.

A developed mind with an undeveloped body would be incapable of truly understanding things that they would otherwise NEVER experience, despite their immortality.

Some things may be extremely difficult or impossible to fully comprehend without personal experience. For example, you could explain the chemical and psychological drive to procreate and its associated physical attraction but that kind of understanding would be limited to observation only. For example, a child might see someone as "beautiful" through visual characteristics but wouldn't truly understand or have an opinion of what is personally "sexy" to them; only what it is to other people.

Here is an interesting list of which you can choose whether you child would be able to experience given their exceptional lifetime while considering others of which they may not:

Additional things to consider is how a child sees the world through their eyes. There are lots of great videos on the subject:

Other references which may be useful:

There is almost always a moment of shock and awe for the unprepared.

I also think it's very interesting that ALL of these biological children and stunted adults are interacted with VERY differently from an actual adult. Whether it's because we perceive them as being immature or less experienced than ourselves, there always appears to be a moment of shock associated either with the individual expressing themselves well BEYOND the observer's perception or expressing themselves well BELOW the observer's anticipation.

The child has complete faculties but acts young by choice, without choice, or as a form of deception.

To take things to another extreme, consider the possibility is that your child may appear biologically young and, while possessing the complete faculties of an adult, CHOOSE to live and act according to the perceived age afforded their physical limitations either because they enjoy it as a lifestyle, because it is demanded by their underdeveloped form, or when it is convenient as a form of deception.

  • 4
    $\begingroup$ This is likely not correct; mind development != brain development and the failed growth modes don't really indicate failed brain development. $\endgroup$
    – Joshua
    Aug 13, 2018 at 4:40
  • 2
    $\begingroup$ The test you mention is called the Sally-Anne test (en.wikipedia.org/wiki/Sally-Anne_test). It is a developmental psychology false belief test, based on theory of mind. $\endgroup$
    – Sam
    Aug 13, 2018 at 6:57
  • 6
    $\begingroup$ @Joshua Citation needed. We can have minds in the same sense that we can have souls, I mean, you can believe in those things if you're inclined to, but from a factual perspective there are only organs. Brain maturity is essential to achieve certain capabilities. For example, puberty makes a complete rewiring of ethics in children. $\endgroup$
    – Rekesoft
    Aug 13, 2018 at 10:29
  • 2
    $\begingroup$ "All knowledge is experience, the rest is just facts." (Einstein) -- I support Zhro's claim that the development of mind and body, in relation to each other, effectively determine the kinds of experience and knowledge a being can gain. I'd add a further example to this -- imagine comparing an immortal child to an immortal adult. How would you distinguish child and adult when beings live forever? Either there is no comparison or something about the behavior and knowledge of the child must differ, for all time. $\endgroup$
    – RoboBear
    Aug 13, 2018 at 18:17
  • $\begingroup$ @Joshua if the brain developed normally in a 1000 yr old, they'd be, well, a 1000 yr old in a young body. Pretty boring story. $\endgroup$
    – DonQuiKong
    Aug 13, 2018 at 20:39

There's a story on this by P. J. Plauger, Child of All Ages. It was mentioned on SF.SE. Here's my summary from my answer:

In the story, Melissa has the body of a preteen girl but is over two thousand years old, fluent in dozens of languages, and is well educated in many fields. She stays alive using a formula that her father developed which preserves life, but it only works for those who have not undergone puberty. Melissa's greatest problem now is finding a way to live as someone who is treated as a legal child with very few rights.

To be more specific, Melissa in the story is presented as very well educated with a general adult outlook on most things, but she has been affected by others treating her as a child.

In the end, she

walks into a movie theater showing cartoons intended "for children of all ages" and decides that that term describes her perfectly.

  • 2
    $\begingroup$ This story obviously confuses the mental age with the body age. This is an example of an adult brain trapped in a child's immature body, it is not a child's mind in a child's body. Given that the same hormones that drive puberty also drive brain development, this would be a conundrum. $\endgroup$ Aug 14, 2018 at 2:56
  • 1
    $\begingroup$ Interesting premise. Thanks for the recommendation. $\endgroup$
    – Aldaine
    Aug 14, 2018 at 14:50

The answer to this is quite subtle and detailed and there is seldom agreement among even the medical fraternity about how much a child's mind is capable of as it develops. Zhro's answer is quite good in that it explains a lot about how the young child's mind learns, but it's controversial insofar as many gifted children learn and behave in a manner that we would normally associate with adults; many are specific cases of Asperger's Syndrome or more generally with the Autism spectrum. My goal is to provide that alternate view.

It is almost impossible to tell what parts of a child's development is a function of prior learning (or lack thereof) and what parts are a function of neurophysiological development (or lack thereof). But, in the case of some gifted children and certainly some (not all) cases of Asperger's, there's a clear case that learning and behaviour can cut both ways; sometimes the knowledge and the ability to learn is independent of physical age, other times, not so much.

To that end, an 8 YO child who's been that way for a millennium will have the depth of perspective one expects from someone that age and will be wise, knowledgeable and reasoned about many intellectual subjects. They will likely have a preferred style of art, music, food, etc. which is sophisticated insofar as it is drawn from a breadth of experience over time. Just how wise, knowledgeable and reasoned that person is will of course depend largely on how willing the person was through that millennium to explore, try new things, read, gain insights through thinking about those new experiences, etc. This is indeed no different to the rest of us. I've met many 'young' 70 YOs and a few 'old' 20 YOs in my time, and the difference usually comes down to experience and curiosity on the part of the individual.

That said, there will be some things our millennial (if you'll pardon the strange use of the term) won't have experienced because of the physical limitations of their body. Some of those should be evident, others not so much. Leadership for instance is something you wouldn't expect a millennial trapped in an 8 YO body to have vast amounts of experience in. This is largely due to the fact that many people would struggle to take a child with a high pitched voice seriously as a leader, regardless of their capability. The pains and difficulties of living in an older body, being a parent, public speaking; the list goes on. Our millennial isn't going to want to draw attention to his or her self for a start, so by virtue of that very fact many experiences the rest of us would take for granted (or at least aspire to) would be missed.

So your millenial is going to have a patchwork of development. Some areas will reveal a much deeper understanding of life, others will show him or her as almost naive.

Of course, this all assumes that the human mind can live that long and continue to store memories. We know for instance that human brains change as a person ages, and about the longest we've known one to function is around 150 years. True we can assume that our millennial has some advanced regenerative capabilities given that he or she has lived a thousand years already, but what we don't know is whether or not that regnerative capability would also impede the laying down of memories. If regneration also supports the brain, then it's entirely possible that our millennial would struggle to remember all but the most significant events in his or her life because the regeneration is wiping out memories to restore the brain. Conversely, if regeneration does NOT support the brain, our millennial may remember everything, but could be brain dead by 200 for all we know.

All in all, there are a quite a large number of variables in play here, but there's nothing to say that a perpetual 8 YO couldn't have a relatively normal development path (given their unique physiology), at least for a couple of centuries.


Methinks an eight year old child that is one thousand years old would be very much like they were when they were eight years old.

Like the op states, the age of 'eight' is more defined by the maturity level of the brain, and if the brain matures no further than that of an eight year old, then the child is, well, an eight year old. If the brain developed any further than that of an eight year old, they would no longer be an eight year old.

That is, for a brain to be a thousand years old, and still be that of an eight year old, there would be no plasticity. No maturity. No building on experience. Every day would be like the last. The brain would be frozen in development. There would be no experiential change. There would be no neuronal development, no brain maturation.

Think Piaget's stages of cognitive development. As the brain grows older, and matures, the brain is able to handle more complex thought processes. An eight year old would be forever stuck at the eight year old developmental level. Otherwise, they would not be eight years old. Thus, every experience would be processed the way an eight year old mind would process it. There would be no maturation of the cognitive approach. The wisdom of a thousand years would be no different than the wisdom of a month.

Of course, the really big problem in human immortality, is in human memory. Our brains do not have infinite memory capacity. I can not foresee any possibility of our brains being able to store one thousand years of memories. So even if the BODY were one thousand years old, could we say that the MIND is one thousand years old? How many years of memories could it store? And what happens when it runs out of storage space? Does it over-write the past memories, or does it just stop recording new ones?

  • $\begingroup$ I like this answer because it brings up a lot of technical/biological limitations a person might run into in their thousand years. Obviously OP's question is purely speculative, but this reponse really highlights how little hard science we have to describe this unique child's situation. $\endgroup$
    – MindS1
    Aug 13, 2018 at 13:23
  • $\begingroup$ My personal opinion on the matter is that, given the limitations of our mind, an immortal would go through life pretty much like a dementia patient - every day would be the same thing, just like the previous day. No experiences added to the next, no new memories formed, but each day would be a clean slate. The mind would long ago have reached peak capacity, peak storage, peak period of synaptic development, peak period of neuronal rearrangement. The mind looses millions of neurons in its normal aging. Eventually it would run out of neurons, or it would be frozen in time. $\endgroup$ Aug 13, 2018 at 13:43
  • $\begingroup$ @JustinThyme We already know that new neurons can be created in the brain, though it's normally relatively rare. But even disregarding that, neuronal connections aren't fixed - just because you remember something doesn't mean you can never forget it. It sounds a lot more plausible that the person in question would have no trouble living in "the now", while losing their memories of "a hundred years ago". $\endgroup$
    – Luaan
    Aug 14, 2018 at 7:50
  • $\begingroup$ @Luaan But the number of new connections is NOT infinite. There is a limit. I strongly suggest that after a thousand years of new connections, that limit would have been exceeded several times over. It would be new connections that would not happen, not old ones. The brain is not like a computer, where new information over-writes the old. $\endgroup$ Aug 14, 2018 at 16:44

It depends on the person. I have webcast technicians in my team as young as 11 years old that are absolutely professional. If they weren't short with piping voices you would think they were adults when they are on duty. I have had a couple of 8 and 9 year olds try out as techs. They learned the skills easily, but they were unable to maintain the necessary focus after the novelty wore off.

I just about died laughing during one webcast when the technical director on duty ripped into one of the camera men for texting his girlfriend instead of operating his camera (live webcast, no second chances). This 12 year old that looks and sounds like he's 10, and whose feet don't reach the floor when he's in the chair, has earned enough respect as a technical director that the cameraman who is almost twice his age accepted the scolding readily.

  • $\begingroup$ "If they weren't short with piping voices". Thank you for this. <3 $\endgroup$
    – Zhro
    Aug 13, 2018 at 2:11

In my own searches, I have noticed a remarkable number of wise philosophers suggest that we should strive to be like children. Of course, they refer to different aspects of being a child, but there seems to be a common thread that children "get it" in a way adults do not.

Accordingly, after 1000 years, ten lifetimes, I like to think this person may be wise. I think it would be enormously fitting if they simply enjoyed the moment, and was a child. (tenses are tricky, but I think I got that right)

But if you looked them in the eye, you would see not only the sparkle of the imagination of a child but... something more. And then it'd be gone. You'd just get to see it for a brief moment, but it might change your life forever.

  • $\begingroup$ That's not really what philosophers say though. They want us to have the clarity that children have. Unfortunately that clarity also comes with irrationality, selfishness, impulsiveness, spontaneous violence, casual theft, and all sorts of other problems - because that clarity is directly linked to not having a mental model of other people as actually being, well, people, or really of the world at all. Anyone who thinks kids are genuinely wise has never seen siblings fighting over who sits in the left car seat, or having a meltdown in a supermarket. $\endgroup$
    – Graham
    Aug 14, 2018 at 16:55

As others have said, we don't really know. Which means that you as a world builder can decide on whatever fits your story best. But one can speculate:

People's behaviour is mostly dictated by society around them. This is also true of children. Your immortal child will behave the way people around them expect them to behave, mostly.

If they are the one and only immortal and have power because of that, they are probably behaving in a very adult manner simply to be taken seriously.

If they are the one and only immortal ruler, but is only a puppet for their advisor(s), they will do whatever their advisor(s) tell them to.

If everybody is young and immortal, they are likely to be much more playful.

Basic personalities are formed while people are very young, though they get embellished later. Your immortal's personality will have been formed during their actual childhood, thousands of years before present of your story. You might want to add flashbacks.

Their brains are not fully developed, so they are probably not smart. This makes the "puppet ruler" scenario rather probable.

Children learn languages very easily, far younger than eight. Your immortal will know all the languages.

Some children pick up social skills quickly, some do not. Expectation is the key, though there is probably also individual differences.

Their motor skills are probably lacking, the relevant brain paths are simply not there.

They will have very many memories, but they will be badly organized due to lacking brain development. They will forget a lot. Even a fully functional adult brain forgets a lot, especially when the years pile up. This child will be worse, much worse.


A 1000 years old human with the body of an 8 year old child would certainly be capable of complex, abstract reasoning because he/she would have had lots of time to allow for the formation of his/her brain. The major difference at the core of both the thinking and the acting of such an old person would be the child-like sexuality.

  • $\begingroup$ Welcome to Worldbuilding, Will! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$
    – Gryphon
    Aug 13, 2018 at 15:09

I'm amazed there are so many answers, and none have really touched on the reality of life for someone with a mental handicap.

My uncle was reckoned to have a mental age of around 4-5, with a side helping of autism. The autistic element naturally made him averse to changes in routine, obsessions over certain things (concern over the bin men wanting to take his TV, for example), but in so many other ways his ability to process and his mental model of other people was the same as a young child. My gran apparently tried to get him work with a local groundsman, trimming hedges and the like, but he would simply wander off home when he lost concentration.

And yet he was able to learn skills. So unlike a 5-year-old child, he could cook, make hot drinks, run a budget after a fashion, and so on. In spite of the 5-year-old brain, he still had decades of life experience to draw on.

I can easily see your immortal working the same. Lots of simple skills, acquired over the years - but no real understanding of what's going on, and still easily tricked or abused by ill-intentioned people.

  • $\begingroup$ But then being a child is not the same as being a child with autism. Leaving fantasy behind, we don't have any theory to make reasonable predictions about a mind with 1000 year experience. My kind of fantasy-based argument is that activity forms the brain, the mind, the personality; therefore we just can't say, what kind of knowledge and skills a child could develop in such a long time .. I dare to say, that it wouldn't be comparable to our concept of childhood, which in reality lasts about 20 years (rather less); I dare to say that that wouldn't be a child anymore, besides sexuality. $\endgroup$
    – user54181
    Aug 14, 2018 at 6:54
  • $\begingroup$ @Will I tried to separate out the autism-like elements. (He was never formally diagnosed on that side of things AFAIK.) The point is that we do have people with child-like processing abilities, but with decades of life experience to draw on for simple skills. Of course we can't extend this into thousands of years, but we have a starting point. $\endgroup$
    – Graham
    Aug 14, 2018 at 7:19
  • $\begingroup$ I was going to propose something similar as an answer, but with the real-life example, you said it so much better than I would have. But yes--take a look at adults with Down Syndrome (Trisonomy-21). $\endgroup$
    – Deacon
    Aug 14, 2018 at 13:01
  • $\begingroup$ Alright, I may be missing the point, kind regards!! $\endgroup$
    – user54181
    Aug 14, 2018 at 14:32

In addition to the plethora of physical/biological/psychological limitations, this person would experience cultural limitations as well - there aren't a lot of situations in which a society would allow a child to just roam free as an adult would. Even a thousand-year-old child would likely be treated as a child for the majority of their life, and as such, may never gain much more knowledge and experience than their 8-year-old peers.

  • $\begingroup$ Bear in mind that most civilizations through history, and even today, didn't isolate and restrict their children quite the way we do in western societies. Even today, a 5 year old Bedouin child can skin a goat in 5 minutes flat with minimal supervision, while we "civilized" folk get in a panic that our child might pick up the knife at all. $\endgroup$
    – pojo-guy
    Aug 14, 2018 at 1:58

Some excellent answers here, but most focus on the biology and psychology of the immortal child.

One aspect of the growth of the mind is life experience.

The immortal child can be born immortal or made immortal later on, and in his/her first 8 years he/she would develop as a normal child.

But once the immortality is discovered (staying the same for 20+ years), his/her life would be turned upside down.

In superstitious times, he/she would be either treated as god or demon. In modern times, maybe a prisoner in a government-funded secret lab, working on breaking the secret behind the immortality. Or living as a fugitive, sometimes sheltered by good-willed individuals. That would force the child to grow up mentally.

I think you might find the behavior something similar to the kids in Akira, or war zone orphans. Only a whole lot more skilled and cunning due to accumulated experience.


Part of you premise is flawed, or at least requires further consideration:

And the brain of an eight-year-old is physically incapable of certain reasoning.

This "physical incapability" (which is not universal to begin with, just statistical) is almost certainly just a consequence of bounds on the rate at which the brain develops and solidifies associations, which in turn acts as a limiting factor against learning things which are wrong. It's quite unlikely (and I'm not aware of credible research suggesting) that there's a deeper physical reality behind it, like endocrine changes that have to happen in order for special parts of the brain to develop with these capabilities.

As such, in order for this sort of limitation to apply to your immortal who's actually 1000 years old in a consistent and believable way, I think you'd need to have some sort of bound on the development of associations/experience/memory. This could itself make for a really interesting story.

Short of that, though, it seems much more plausible to me that an immortal who eternally looks like an 8 year old has to put serious active effort into behaving believably like an 8 year old.

So, to answer the question, I think either:

  • They'd behave like a really experienced adult in a child's body, probably trying very hard to hide it, or
  • They'd behave like someone with the inability to grow mentally in most or all ways, including development of new memories.
  • 1
    $\begingroup$ The brain is a sexual organ and goes through a developmental stage based on the sex hormones released during puberty. If the child did not go through puberty, these changes would not occur. It would be distinctly a child's mind. fasebj.org/doi/abs/10.1096/fasebj.25.1_supplement.187.4 $\endgroup$ Aug 14, 2018 at 3:15
  • $\begingroup$ Sorry, but the brain of an 8-year-old is physically undeveloped. This is why severe emotional trauma in young children can be seen physically in scans of the brain, due to the way in which impacts neurological development. $\endgroup$
    – Deacon
    Aug 14, 2018 at 13:05
  • $\begingroup$ What does "physically" mean? The storage of memories, experience, acquired skills, etc. in the brain is all physical. $\endgroup$ Aug 14, 2018 at 17:18

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .