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