In the eyes of evolution, individuals lose their purpose after they reach the end of their reproductive lives. After the creation of offspring, natural selection has no further role to play for the aging individual. If you've had kids and aren't going to have any more, your death has no effect on the genetics of your population anymore.
Humans, though, do live past their reproductive prime, and more so than many other animals. Evolutionary biologists have argued that the potential for old age in humans has to do with two things. One, men never lose their reproductive potential, and two, post-menopausal women can serve to aid in the rearing of their children's children, thus facilitating their own biological fitness, albeit in a round about way. But this potential for old-age extensions of fitness was not strong enough as a selective pressure for our bodies to have evolved to maintain full functioning in old age. Young people in their prime who died early were still typically more fit than those individuals who survived to extreme old age.
In this article, from a 2001 issue of Scientific American, the authors look at the human body from a mechanical perspective. They evaluate the most common issues associated with aging, such as arthritis, hearing loss, and vaso-constriction, and propose "fixes" inspired by comparative biology and engineering. It shows how evolution could have resulted in a completely different "human" if selective forces had favored long-life over early reproductive success.
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