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In plenty of stories you have beings who age incredibly slowly and are technically immortal. I have a handful of similar such beings who have through some means or other attained the benefits of healing blood which lead to their 'immortality'.

Now, I've not made these people unable to age at all. They do age, just incredibly slowly. So eventually they will grow old, and eventually they will die. Probably something to do with a combination of their cells telomere lengths and the healing aspects of their blood thinning out... but they will live for an incredibly long time, a couple of centuries to a millennia or two at my last count. And seeing as the fundamental role of life is to go forth and multiply, I have allowed them to have children, albeit very rarely and very slowly. It's actually quite a sad affair involving normal 'mortal' women, as immortal women do not have their period for centuries. The universe is not going to get overrun with immortals.

My problem is, how do I get the newborn immortals to grow into adulthood without subjecting them to several decades to centuries worth of childhood and worse, puberty. Puberty is tough enough as it is without being stuck in it for decades. I don't mind them taking several years to age a 'normal' year but I want to avoid decades to age a 'normal' year. I am trying to avoid the 'wise child' cliché, as well as resorting to birthing a fully grown adult. As in 'The Birth of Venus' or Athena.

How can I explain why my immortal children age faster than the immortal adults?

extra info:

Their bodies heal all wounds and infections, their cells renew continuously without cancer forming. If a limb is chopped off, they can reattach it (if quick enough) but they can't grow it back. And if they break their bones and let them set in the wrong position, they will heal in the wrong position. Stuff like that. I'm not too worried about this aspect of their immortality as it is pretty well covered in many stories and even other questions on SE.

I've figured a way to ignore/handwave away the whole 'pregnancy is essentially an infection' issue by making the immortal women sterile. (EDIT actually they are fertile and develop as any normal female, their bodies immune system is just so keyed up that it rejects all foreign genetic material and either never conceives, or aborts the foetus very very quickly. END OF EDIT). 'Normal' women with some limited exposure to the source of the immortality (but not themselves immortal) are involved and the foetuses develop over several years rather than 9 months. FYI this is more an incubation in an external pod type pregnancy and there are no hormonal women walking around who have been pregnant for several years. It's also not about extended growth inside a womb but rather just very slow foetus development inside an artificial womb made from the same source as the adapted/healing blood. So they are still born at a similar development stage as a normal human baby due to normal energy and child development constraints etc. Again, I'm not too worried about this aspect either, unless I should be.

No magic, no technology, just adapted blood and a naturally grown birthing chamber/cocoon/pod like a silkworm cocoon or leaf pod.

I'm looking for a logical and semi reasonable explanation for increased rate of aging for 'immortal' children compared to the much slower adult aging rate.

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    $\begingroup$ I think its funny that immortals would probably have to take lessons and practice setting bones quickly. I wonder if there are any other things liike that, that would be unique to immortals? $\endgroup$ – Necessity Jan 29 '17 at 23:56
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    $\begingroup$ That's a bit mean on the immortal women. How about they have a period every x years, and can get pregnant then. Plus, while they are pregnant they lose their immortality, then age at the same rate as everyone else. After the birth they get it back again, or after lactation. This would mean they can if they want, but it's a nuisance. $\endgroup$ – RedSonja Jan 30 '17 at 9:06
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    $\begingroup$ @RedSonja I actually like the idea of getting it every x years and was pondering it just before your comment after I worked out the ratios etc. A bit tongue in cheek but like a cat coming into heat :) $\endgroup$ – EveryBitHelps Jan 30 '17 at 11:33
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    $\begingroup$ Simply, "growing up" is a continual increase in physical and mental capacity to a "peak" of physical and sexual maturity, where "ageing" is slow decline in physical and - after a point - mental capacity due to damage, essentially. Therefore any therapy that slows the rate of damage or "ageing" need not have any effect at all on the rate at which people mature. $\endgroup$ – Grimm The Opiner Feb 1 '17 at 13:56
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    $\begingroup$ Humans stop aging around 90. Once you hit this point you're as "aged" as you're going to get. After that your system only degrades because the wear piles up faster than your aged system can handle. If this switch happened earlier, say in your 30s, you're system would be able to keep up with the damage well enough to keep you from dying in most cases, making you virtually immortal which is what you're talking about so i don't see why this would be an issue. You can just say that the people age after 30 only because they have sustained too great of wear to their body for a given period. $\endgroup$ – Durakken Feb 26 '17 at 18:58

11 Answers 11

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Immortality is part of puberty

You can have children mature at a normal, human rate, which may not be "faster" but it will appear that way relative to their adult counterparts.

Then, when puberty hits, their body can start the "immortality process" along with every other change. Puberty already employs a host of changes that you can use as an excuse for changing the body. It causes widespread growth, develops and changes many existing cells, and releases hormones that were previously not present. Any of these processes could release whatever you need to cause immortality - and slow down aging - just handwave and say the body knows how to make the right chemicals.

It would also lead to some interesting circumstances involving children and infants - they'd still be completely vulnerable, so they may be more sheltered.

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  • $\begingroup$ This was my first thought and seems like the least possible amount of handwaving. $\endgroup$ – Joe Jan 30 '17 at 12:42
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    $\begingroup$ @Joe any handwaving that was necessary to explain immortal's infancy was already applied by assuming the existance of immortals. Everything else is part of the bonus package. $\endgroup$ – Mindwin Jan 30 '17 at 13:23
  • $\begingroup$ To tie this in a little further, immortality could be a byproduct of a specific gland that is activated at puberty in immortals. The hormones secreted through this gland could also be the key to their hyper efficient immune system. Regular humans would have this gland in a vestigial state but it never activates. Depending on the story being told this opens interesting angles on a single random mutation being the origin of immortality or immortal specific diseases like fountain-of-youth gland stones. $\endgroup$ – Myles Jan 30 '17 at 15:10
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    $\begingroup$ @Myles, so that is what the appendix is for! Wait, Sorry. Suddenly had an image of people hunting for immortal's tailbones. ''Ground up tail bone, a sure fire cure all and guaranteed to make your night longlasting!'' $\endgroup$ – EveryBitHelps Jan 30 '17 at 17:20
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    $\begingroup$ @EveryBitHelps I'd go away from a well known part and more like a pea sized gland in the head or neck that only pops onto human radar when it infects or goes cancerous. $\endgroup$ – Myles Jan 30 '17 at 17:45
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Because why not?

There's no actual scientific requirement which forces your immortals to age proportionally, and there's plenty of scientific reasons for why they would not.

Growing up is not a random set of changes. As much as the poor pubescent boy might despise his cracking voice, everything that happens to children as they grow up happens for a reason. Grow up too fast and you are under-prepared for the rigors of adult life. Grow up too slow, and you reproduce too slowly. Nature is constantly seeking that middle ground and balancing it.

There's no reason to assume your children grow up at a proportional rate to the adults. It'd be like arguing children can't go through growth spurts because most of the time they're growing slowly.

Now the real question would be why would a particular length of childhood be chosen. That's up to you and your immortals. If the immortals evolved, you'll want an evolutionary reason why that particular length of childhood is used.

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    $\begingroup$ @EveryBitHelps Biologically, childhood ends with puberty. At that point the growth spurts stop, and you are capable of having children. Biological childhood is based on the reality that it takes time to grown bones and muscles correctly, and more importantly, you need time for the brain to learn how to use them. Social childhood is based on how long it takes to have enough mental maturity to be considered responsible for your own actions. That could vary for the immortals in any way you see fit, based on their culture. $\endgroup$ – Cort Ammon Jan 29 '17 at 23:32
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    $\begingroup$ @EveryBitHelps Even in reality, some cultures say adulthood is 13, most governments that I know of say 16, 18 or 21, and scientists are now saying the brain isn't done developing until around 25. I still get 'son' and 'kid' from people that are twice my age, and probably will until I'm the oldest person on earth. It's up to you to decide when the Immortals figure that 'childhood' is over, or even whether it really matters to them? Perhaps they don't just have a 'childhood' and 'adulthood'? $\endgroup$ – Robotnik Jan 30 '17 at 1:47
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    $\begingroup$ @Robotnik no need to be the oldest on earth - halfway is enough to stop people twice your age calling you anything ;) $\endgroup$ – Baldrickk Jan 30 '17 at 9:58
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    $\begingroup$ @Robotnik it's just math :P $\endgroup$ – Baldrickk Jan 30 '17 at 10:05
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    $\begingroup$ @ohwilleke There's some evidence that humans are capable of reaching sexual maturity a lot earlier than they do in modern societies. Primitive african tribes still do mature a lot earlier than "us" - some think it's caused by different food patterns, and to some extent, culture. And you can't compare humans to cats - humans are rather large as animals go, and we know that bigger animals take longer to develop (though it certainly isn't the only factor - e.g. herbivores mature faster than carnivores etc.). Both dogs and cats have also domesticated - they are less mature than their ancestors. $\endgroup$ – Luaan Jan 31 '17 at 15:14
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Well, you could say these are two distinct types of "aging". The "aging" in adults is more or less a maintenance process. Cells that are burnt out will be replaced by new cells. Except for a few body parts and specific cell arrangements like taste buds, the neurons in the hippocampus and a few others, the human body does not grow after a certain age (which differs from individual to individual to a degree). This process of cell division, cell life, cell death and renewing is (very much simplified) aging. Signs of aging will also develop if the body is not able to repair the damages through environment fast enough, or if the DNA is damaged by those factors, for example through radiation or toxins.

Children will also grow at the same time, to develop the features of a mature individual of the species. The splitting and growing processes of the cells are much more rapid than in an adult, so the child can mature. Children also heal wounds and broken bones much faster than older individuals.

This is the reason why immortal children age faster until they are biological adults, at which point they will have the same slow aging process as the other adults.

You could read up on Wikipedia Ageing and Wikipedia Cell Cycle and see if you can find something "harder" (scientificly).

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    $\begingroup$ to put it more succinctly, the immortals would mature, but not senesce $\endgroup$ – stellatedHexahedron Jan 30 '17 at 16:50
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Keep it normal

Well actually it would make perfect sense for them to grow up at the exact same time frame as normal people, if all you want for your immortals is for there cells to continously heal themselves perfectly or create perfect duplicate cells to replace them. Which is actually the reason we cannot live longer because our bodies can't do its processess perfectly.

As long as your immortallity doesn't affect the normal release of growth hormone, then they will just continue to grow until 21(ish)(the age most people stop growing at the latest) and then none of the cells in there body will deteriorate. Preventing them from "ageing", remaining in a youthful state for a long time.

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  • $\begingroup$ In this case, they would 'appear' to be in their baby-faced twenties the longest? this actually sort of lines up with what I have at the moment. $\endgroup$ – EveryBitHelps Jan 29 '17 at 23:37
  • $\begingroup$ @EveryBitHelps Yeah but honestly I find it harder to explain why immortal women are infertile and babies stay in the womb for years, even if that is a common trope. $\endgroup$ – Necessity Jan 29 '17 at 23:45
  • $\begingroup$ well in my case, it's not that the women are exactly infertile it's just that any pregnancy would be aborted by the 'healthy' body as an infection. And as for the babies, I didn't want a women pregnant for years, so I took the whole pregnancy away from them and put the foetuses in those pods... so they can have their slower growth/development/aging. is that more helpful? $\endgroup$ – EveryBitHelps Jan 29 '17 at 23:55
  • $\begingroup$ @EveryBitHelps Oh, I underatnd. I just disagree with the trope of preganancy is an infection. It's a ridicuolus idea. $\endgroup$ – Necessity Jan 30 '17 at 0:03
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    $\begingroup$ @EveryBitHelps It is a trope because it is a metephor for alot of complicated reasons as to why the womens body rejects pregnancy. Normally immortals cant get pregnant not because 'Pregnancy is an infection', rather because there bodies are in a frozen state, it cannot change into a body capable of going through pregnancy. $\endgroup$ – Necessity Jan 30 '17 at 0:26
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{handwavium alert}
The reaction kinetics of the telomere-reconstruction enzyme used, imposes asymptotic aging toward the pre-defined ideal apparent age.
{/handwavium alert}

(Please bear with me, I haven't learned the math formatting here, and my Tex foo is old and rusty.)

Take, for example, males in some (~western) society, for which the optimal age for immortals is (you guessed it), 42 years.

d(age_apparent)/dt = (Age_optim - age_apparent)/Age_optim

{classic first order ODE; exponential decay toward asymptote}

at birth, a male immortal ages at a rate of one apparent year per calendar year. At age 21, he ages at half an apparent year per calendar year. He never quite gets to (an apparent age of) 42.

However, relative ages of immortals are preserved -- except for those older than 42 at treatment time; those folks regress (per the formula) asymptotically toward an apparent age of 42. Note that (as required) the infant ages faster than the 21-year-old, and a 41 year old ages at an apparent rate of just 1/42 of the newborn.

Empirical cultural research strongly suggests that the corresponding age for immortal females would be 29. (But cultural research is notoriously dodgy.)

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    $\begingroup$ I have only the vaguest idea what you just said but I love it :) $\endgroup$ – EveryBitHelps Jan 29 '17 at 23:06
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    $\begingroup$ I have only the vaguest idea what you just said and I'm not sure what to ... make of it? I get the joke involved but even still what just happened $\endgroup$ – Zxyrra Jan 30 '17 at 0:13
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    $\begingroup$ @JDlugosz, but that's why I mentioned a combination of telomeres AND healing aspect. So for the longest time, the healing aspect prevents cell damage in the first place. Then when cell damage does occur, it helps restore it correctly. Then if and when cell damage occurs too widespread for the healing to occur immediately, the damaged cells can be inhibited from replicating too fast. Something like, 'prevention, restoration and inhibition'. Obviously this idea still has flaws in it, as I'm not a medical expert, which is why it's not the focus of the question :) $\endgroup$ – EveryBitHelps Jan 30 '17 at 8:14
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    $\begingroup$ tl;dr: Some biological mechanism keeps immortals at their apparent age, so it's reasonable to expect that same mechanism to push young immortals up to that same apparent age. Hah it's a clever point! $\endgroup$ – Nat Jan 30 '17 at 10:59
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    $\begingroup$ This answer reminds me of 8-Bit Theater, Comic #861: In the Family, in which White Mage explains that her magic works by reinforcing the body's natural state. In this case, an immortal's natural state would be whatever their equilibrium apparent age is; so, their body would tend to "heal" toward that age, whether they're younger or older than it. $\endgroup$ – Nat Jan 30 '17 at 11:08
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As many answers on here have pointed out, puberty can be the trigger for the start of the slow aging. Here are some alt triggers that might help.

Max Bone Density

You could link it to peak bone density something that is largely determined by genetics. What this means is that the slowing of age will vary, but it will significantly older than puberty, which can hit as early as 11 years of age for some women.

They don't sound remotely human, but this is something you can link it to--and that tends, in human populations, to be in the 20s or even in some cases the 30s, with a few outliers in the late teens and early 40s.

A Specific Brain Development

Science tells us that our brains don't fully mature until about 25 years of age. For some it's a little earlier, for others a few years later, but 25 is the average.

In this case, you'll link the immortality kick in to a specific development in the brain, which will then signal the rest of the body to change to the slower aging.

Further, the quick healing likely should not kick in until they gain their immortality, leaving them as vulnerable as regular folk until they come into it.

Slowing of Myelin Production

The production of the myelin sheath is called myelination or myelinogenesis. In humans, myelination begins early in the 3rd trimester,1 although little myelin exists in the brain at the time of birth. During infancy, myelination occurs quickly, leading to a child's fast development, including crawling and walking in the first year. Myelination continues through the adolescent stage of life. [source]

This is connected to brain development and also nerves and spinal cord.

Collagen Production

Collagen levels peak at the 20s and then begin to fall. As it does, the immortality genes or whatever your mechanism is, kicks in to compensate.

TO SUMMARIZE The crux of this and all the other things on this list is this: the minute the mind and body stops growing and begins to age instead--that is, the developmental clock is no longer going, and is simply aging, the mechanism for immortality kicks in, like a switch being flicked. I'd look at it as a form of specialized epigenetics, wherein a particular series of bodily changes triggers it. It doesn't even have to be ONE thing on this list--it can be all of them!

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I would suggest that your immortal children would actually age faster than normal. You can think of the aging process as a chemical reaction and your healing blood is a catalyst that drives it along faster than normal. All the hormones put in our blood as we grow up would be used up faster and that would trigger more hormones to be released, until the body gets to its equilibrium.

Another way to look at it is that normal human bodies utilize as much of the resources they take in as possible, which often leads to overweight individuals. On the other hand, if your healing blood is capable of turning calories into healing (which basically has to be the case due to entropy), it wouldn't be a stretch to say that immortals that haven't yet finished growing would end up burning off all their excess fat to grow faster.

One of the major side effects of this would be that most immortal children would never be overweight. Regular exercise would be capable of building muscle, as normal, but body fat would disappear rapidly. Additionally, since those children would be always hungry, normal parents would probably feed them too much and they would grow up faster than normal. Conversely, if an immortal child was starved, their healing blood would probably slow down their rate of aging, in an attempt to preserve the body in the current state rather than breaking down significant amounts of it to fuel growth.

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There is a distinction between ageing and growing. There isn't much of a consensus on what causes ageing but I consider it a degradation of cells. The symptoms of Progeria are actually caused by structurally unsound cells.

On the other hand, growing is the natural change in your body. These changes are regulated by hormones which basically culminates in puberty. A quick search shows that cartilage never stops growing but bones stop growing after puberty. Interestingly muscle and fat cells generally stop splitting after puberty.

Growth, especially fetal, is measured by the speed in which cells divide. In pregnancy the general rule of thumb is "doubles in weight every week--until the second trimester." Fetal development is ultra-complex and considering the above paragraphs, I'd regard survival of a "long" development time would be improbable.


In the Hebrew Bible (Old Testament), the genealogy has several records of very old humans, such as Methuselah (969 years old). If you take this literally, the idea is very similar to yours. Except instead of magic blood it's theorized that diverse genetics, limited solar radiation, and a highly supportive environment & atmosphere contributed--or failed to contribute to--the ageing process. The Old Testament also assumes that humans were made immortal to begin with, and it's only after "malfunction" was introduced that the system failed. Then the environmental constraints that supported long-life were reduced by the world-wide flood. The diversity of genetics were also reduced by mass-extinction. Interestingly sea-life would be less effected, which could explain creatures like the immortal jellyfish.

A big, unanswered question, is how puberty worked in the pre-flood era. For the genealogy concerning Methuselah, the earliest child was age 65. Methuselah had his first child 20% (187 years) of the way through his life. Currently, puberty at 13 and life expectancy of 78 would yield a "best-case" of a child at 16% of my lifetime.

This leads to a variation of your exact question: did pre-flood parents change exponentially more diapers than post-flood parents?


Based on the distinction I made between ageing and growing, fetal development, and child-bearing age of Old Testament pre-flood guys, I conclude that babies developed into youth at a similar rate but hit puberty much later on in life.

To answer your question, have them "grow" at a natural rate but "age" at a reduced rate.

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    $\begingroup$ Welcome to WorldBuilding! Did you notice the tags on the question? Questions with "science-based" are specifically looking for citations using validated data. Using Genesis as a historical document is a poor choice, in line with citing that aliens can probably crossbreed with humans because Zeus was able to spawn demigods with human children. The Bible is an excellent basis for a lot of historical questions once it reaches the New Testament, but there's very little that survives peer review in Genesis. $\endgroup$ – SRM Jan 31 '17 at 4:55
  • $\begingroup$ Hey SRM, thanks for the comment. As I'm new to worldbuilding, I didn't notice the tag. I'll keep that in mind for the future. My intention was the first part would be modern science on growth and ageing. Then the second part--the Old Testament--would pose a nearly identical question--not intended to be a scientific answer but a parallel. Finally, to conclude with answering both questions. After a quick review, there are several updates I could make to clean-up the format. I will be updating my answer when time permits. $\endgroup$ – Nathan Goings Jan 31 '17 at 6:31
  • $\begingroup$ no prob. That's why we flag new posts ... to let people know about the conventions of the site. :-) $\endgroup$ – SRM Jan 31 '17 at 6:45
  • $\begingroup$ @SRM, I do get what you are saying but I still consider it a interesting answer. While I do take the Bible with a very large grain of salt, and a bit of lemon too, it is still an interesting answer trying to use modern day scientific definitions of growing and aging to explain the fantastical elements that exists in existing mythological lore. $\endgroup$ – EveryBitHelps Jan 31 '17 at 6:54
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For "normal" humans, aging is a positive thing until they hit an age of about 27-28. After that age, the body starts deteriorating. So it would make perfect sense for the healing blood, to embrace aging until the body hits its peak, and only then start to prevent (or slow down) the aging process.

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Why would they age slower than normal humans?

Basically humans grow (I didn't say "age") until sexual maturity, and when it's completely done, after they've peaked, the body slowly decay. If we didn't age we'd still reach adulthood at the same age, we'd just look 20-25 for much, much longer.

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Babies and children are not aging - they are maturing. Growth and maturation is a biological process with a specific end result: to reach physical, mental and sexual maturity. Think of this stage as analogous to building a house or manufacturing a car.

Aging is the wear and tear and inevitable breaking down of biological systems after they have finished the growth and maturation period. It is not so much a biological process as an accidental side effect of many biological processes. Think of this stage as analogous to an old house's roof leaking or your car's tyres getting worn because you've driven thousands of miles.

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