How could immortal children age faster than immortal adults?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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).

Answer 4

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.

Answer 5

{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.)

Answer 6

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!

Answer 7

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.

Answer 8

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.

Answer 9

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.

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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?

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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.

Answer 10

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.

Answer 11

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.