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.