The comparatively long length of the human life span (nowhere near long enough if you ask me, but a good deal longer than other mammals of similar size and weight) poses an interesting problem for evolutionary biologists. You can get a flavor of current thinking on this issue from a recent Tom Kirkwood paper:
Evolutionary considerations suggest aging is caused not by active gene programming but by evolved limitations in somatic maintenance, resulting in a build-up of damage. Ecological factors such as hazard rates and food availability influence the trade-offs between investing in growth, reproduction, and somatic survival, explaining why species evolved different life spans and why aging rate can sometimes be altered, for example, by dietary restriction. To understand the cell and molecular basis of aging is to unravel the multiplicity of mechanisms causing damage to accumulate and the complex array of systems working to keep damage at bay.
So how, we might ask, did humans end up with long life spans? One set of theories revolves around selection effects that stem from the benefits of socialization and society.
Details of how longevity increases over the course of human evolution provides a wealth of information on how human social networks developed.
The number of people living to older adulthood would have allowed early modern humans to pass down specialized knowledge from one generation to another.
Old age would have also promoted population growth and strengthened social relationships and kinship bonds.
One fairly detailed theory is known as the Grandmother Hypothesis - that prolonged lifespan has been selected to increase the reproductive success of offspring. In other words, the ability to help your grandchildren do well is the driving selection mechanism, not the ability to help your children do well. Enough thought has gone into this for an entire book, it seems:
Darwinian theory holds that a successful life is measured in terms of reproduction. How is it, then, that a woman's lifespan can greatly exceed her childbearing and childrearing years? Is this phenomenon simply a byproduct of improved standards of living, or do older women - grandmothers in particular - play a measurable role in increasing their family members' biological success?
Until now, these questions have not been examined in a thorough and comprehensive manner. Bringing together theoretical and empirical work by internationally recognized scholars in anthropology, psychology, ethnography, and the social sciences, Grandmotherhood explores the evolutionary purpose and possibilities of female post-generative life.
While I'm not sure than any of this directly bears on strategies for extending the healthy human life span via improved medicine, it is interesting.