Up to 35 percent of your life expectancy is due to the genes you inherit. The rest depends on what happens after you're conceived. One of the things that happens is that there are errors and mistakes when your cells copy your DNA. Most of those errors are fixed or destroyed by the body, but some slip through. Over time these errors add up and begin to interfere with the normal functioning of the body. At least that is what the somatic mutation theory of aging believes.
The somatic (body) mutation (change) theory believes that change in your genes AFTER you are born create problems and lead to aging. While this theory looks to genetics to explain aging, these changes are not hereditary because the change occur after your original DNA is created. These changes can be caused by exposure to toxins, ultraviolet light, random change and other factors.