On a broad scale, longevity in animals approximately correlates with body size,
which correlates approximately to metabolic rate. At either end of this
spectrum you find animals such as doormice and elephants with short and long
life-spans respectively. However nothing is that simple in nature. Contrast
the mouse and the bat, both are mammals with approximately the same body size
(although with very different life-styles). The mouse lives about 3 years and
the bat for over 30. Clearly lifestyle is important.
Genetics also seems to have a role to play. In the nematode Caenorhabditis
elegans, genes have been located, which if mutated can double the life-span of
In humans, and other organisms, we seem to AGE until a certain point (late
20's) and then we undergo not aging, but SENESCENCE. This is the gradual
deterioration of the body (the big downhill!). One theory (PLEIOTROPY) for this
is basically that we have good genes in our body that are selected for, but are
linked to bad genes which cause senescence. However as the effects of these
bad genes occur late in life, the early-acting good genes are selected for
carrying the bad genes along for the ride. However, I'm not sure if their is
any experimental evidence for this.
If there were a genetic component to aging in humans, and a gene/combination of
genes were found, then I would wager that some amoral drugs company would try
to patent the gene. You can just imagine drugs called "Xtralife" can't you?