The genetic theory of aging believes that lifespan is largely determined by the genes we inherit. According to the theory, our potential age is primarily determined at the moment of conception.
There is some evidence to support this theory. People with parents who have lived long lives are more likely to live long themselves (though this could be partially explained by learned behaviors, such as food preferences). Also, identical twins (who have the exact same genes) have closer lifespans than siblings.
Some genes are beneficial and enhance longevity -- a gene that helps a person metabolize cholesterol would reduce a person's risk of heart disease, for example. But some genes are harmful, like those that increase the risk cancer. Some gene mutations are inherited, too, and may shorten lifespan. (Mutations also can happen after birth, since exposure to toxins, free radicals and radiation can cause gene changes.)
It is estimated that genes can explain a maximum of 35 percent of lifespan. The other determinants are your behaviors, exposures, and just plain luck. So don't think that you are doomed just because your family members tend to die young -- and also don't think that you can ignore your health if your family members tend to live long.
Sources: How Do We Age?. The American Federation for Aging Research
