Some scientists believe that the accumulation over years of uncorrected DNA damage is a major cause of aging. They cite the following observations:

Animals with the fastest rates of DNA repair generally have the longest life spans.
Animals with the highest rates of oxidative damage by free radicals (and specifically, with oxidative damage to DNA) generally have the shortest life spans.
In lower life forms subject to oxidative damage, anti-oxidant supplements, which can correct and prevent DNA damage when it occurs naturally, do increase life span. This has been shown in worms, insects and rats.
Exposure to external causes of DNA damage (ultraviolet light, tobacco) decreases life span.
Humans who have genetic diseases resulting in greater spontaneous DNA damage or inefficient DNA repair often show signs of premature aging.⁷

Evidence exists for the decline in DNA repair and the accumulation of DNA damage in several different types of cells taken from elderly subjects. Elderly patients' blood⁸ and skin cells⁹ have less capacity to repair themselves than those from young adults. Indeed, one study that looked in white blood cells found DNA damage in 2-4% of the cells from young adults, but six times more often in cells from the elderly.¹⁰ These aging white blood cells with their higher level of DNA damage may explain some of the decline in immune function associated with aging.