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There's a species called Foos. Foos are fairly similar to humans. In a primitive society, in fact, they would appear identical to humans in actual physical appearance, mannerisms, and base behavior. The one distinguishing feature is that between the ages of 25 and 35 (approximately) they stop showing the signs of biological aging externally (both in physical appearance, and biological behavior, such as athletic ability and mental acuity). They still suffer from wounds, starvation, etc.

Eventually, just like humans, they eventually die, with the average age span comparable to humans.

Now comes in science to study why Foos don't see to age. Well it seems they don't have a liver or kidneys, and in their place have a singular organ called a foobar. The foobar serves the purpose of both the human's kidneys and liver, but is slightly more efficient and considerably more resilient. Further, it produces a cocktail of chemicals that it releases in the bloodstream that serve to slow (virtually stop) aging, the cocktail varied to fit the day-to-day chemistry of the body, and also very unique to each individual (like a more extreme version of blood types). These two factors combined make transfusion of the cocktail impractical to the point of impossibility. Similarly, donating foobars is nigh impossible.

The reason for the specific age range is simply when the organ reaches maturity, a similar idea (if for completely different reasons) to puberty. The environment and genetics would play into exactly when the organ would reach maturity.

It turns out that, after so many years, the foobar fails to keep itself going, and eventually starts to fail, typically killing the Foo. It should be said that the foobar isn't perfect; the organs (everything that benefits from the flow from blood) its cocktail supports can still fail for a variety of reasons, so the Foos can also die naturally in this manner.

This question is two fold: How reasonable is this idea, and if unreasonable, what can be changed about it to keep it as intact as possible? Preferably I would like to keep the external social consequences similar:

  • Primitive societies could only tell they were different from humans because the halt of aging (I assume, also, the inability to mate or at least produce fertile offspring with humans? I'm indifferent on this point)
  • They do eventually naturally die
  • The medical ability to extend the halt of aging is limited
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    $\begingroup$ Seems reasonable to me. We already have species on this earth that do not suffer senescence. Further, there's several theories for human aging and the "cocktail organ" is just vague enough to fit into all that I'm aware of. A bigger biological problem might be explaining the evolution of this unique organ. Indeed, scientists are still somewhat baffled at why some species never age, while most others do. $\endgroup$ – fredsbend Jul 12 '16 at 16:39
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There are three ways in which we perceive aging in humans:

  • Visually, we see noticeable changes in the skin, teeth, nails and hair.
  • Physiologically, we see that people generally tend to lose stamina with age (this is far from being an absolute, though).
  • Verbally, they start using expressions like "get off my lawn" and "when I was your age..."

The third one is mostly cultural. The second one we perceive aging can be gamed with - if you live a healthy life - healthy diet, exercise a lot - your stamina will decrease more slowly, and may take longer for the decrease to even be noticeable.

What we can't do without cosmetics is halting the external signs of aging. Those have to do with:

...and wear to some extent. There is a lot more involved, but in order to get to the whole science of it one needs a degree in medicine and speciaization in dermatology, so I'll keep to the things I've already mentioned.

Say the Foo people are quite like humans, but the foobar gives them a power of regeneration somewhat stronger than what we humans have - not a healing factor like Wolverine's, but they do not get permanent scar marks, for example. Their teeth and hair also last a lot longer.

Perhaps they have really long telomeres.

Perhaps they produce more collagen, or a different, more resilient variety.

Perhaps instead of accumulating toxins in every tissue like we do, their foobars allow them to keep toxins - and specially free radicals - contained only in those non-human organs they have. They look young outside but if you could take a picture of the foobar of an old foo you could use it to freak children out.

So say that all other tissues are free of toxins because the foobar is so good at taking toxins from other organs and into it. At some point it will reach a threshold and fail. This could even mean cancer. Maybe old foo all die of cancer because their foobar just can't take any more poison. Maybe the foobar stops working all of a sudden and all the toxins accumulated attack and consume the rest of the body in a single attack wave.

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The organ sends chemical signals to all cells in the body, changing how their telomeres behave and causing them to start or stop splitting as needed.

Why this would evolve in any given species is more mysterious. Most species that do age and die do so because, probably, it is evolutionarily advantageous for them to cycle individuals in order to generate more genetic turbulence in a given population. Other species that do not age can't generate that turnover through death due to extenuating circumstances (their gene pools are always scarce enough, or they always are killed soon enough, that aging is not an advantageous trait). Your species should ideally have an evolutionary reason for evolving alterable aging.

One plausible option which presents itself is the idea that such a species would need to keep reproducing throughout much of a very long adult life in order to guarantee successful reproduction. Animals that have slow gestation and lengthy childhoods do not have many children because they focus on fewer, higher quality offspring. But what if those offspring stood a decent chance of dying prior to reproductive age regardless of upbringing? Conceivably, an adaptation could arise which kept reproductive adults at peak fitness for a long time, so that they could mate and raise children many times, increasing their chances of successfully reproducing.

It would also help if there was something else keeping nature from "finding" the more common solution, which is to just evolve so that many offspring are created and not a lot of work is put into keeping them alive. This could happen if the reproductive structures of the animal were too uniquely adapted to create single, high-quality offspring with long gestations.

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