I was told verbally by a biologist that the death rate from disease, predation, accidents, etc. is so high that there would be little advantage to an animal that did not age. He said that a professor he had worked under had discovered this. The professor did his experiments on fruit flies.
That sounds right for fruit flies. It could even be part of the story for the human race. However, humans and many other animals are relatively immune to predation, at least compared to fruit flies. So for humans, elephants, tigers, and other animals that rarely suffer predation an extremely long reproductive life could increase the number of offspring. So perhaps there is another reason why we have menopause, aging, and death.
Frequently venereal diseases cause infertility rather than death, for example, gonorrhea and chlamydia. This is probably because the transmission of the disease involves the reproductive organs, so the reproductive organs and therefore the reproductive function tends to suffer from venereal diseases. Of course, this is only a tendency, venereal diseases do spread to and effect other organs of the body.
So before modern medicine the older organisms might frequently be infertile. Therefore they would not produce children, but they would spread the venereal diseases that produced infertility. So the old might be a net drag on the reproduction of the species. What is more, they might reduce the fertility of their own relatives. Genes that caused us to age and die might help us have more descendants, because they might prevent us from contributing to the infertility of our offspring. So animals with the genes for aging might ultimately prove more fit and push the relatively immortal individuals out of the gene pool, into extinction. Perhaps this is one of the reasons we observe aging.
I thought of this as a result of watching a stupid TV show, "The Highlander." Watching stupid TV shows is not generally a very productive way to pursue scientific knowledge, but it does have the advantage that the resource is abundant, because people spend a lot of time watching stupid TV shows.
In "The Highlander" the hero is one of a race of immortals that do not die unless their heads are cut off. The TV episodes revolve around sword fights that end with one immortal lopping off another's head and absorbing his power. Another major feature is the hero, Duncan MacLeod, jumping in and out of bed with various women. The immortals are infertile or at least do not produce other immortals. It occurred to me that after hundreds of years of bed-hopping the hero must have and be spreading every type of venereal disease. So he was not just infertile, in-spite of being very virile, he must be spreading many venereal diseases and thus making many of his conquests infertile too. Of course the immortals are to some degree magical beings, so the normal laws of biology do not necessarily apply. Nevertheless, this is where I got the idea.
Sex is frequently about reproduction. Sometimes it is the reproduction of us, sometimes it is the reproduction of our venereal diseases, but it frequently involves reproduction, regardless of our objectives. But I digress.