In a self-repairing system, a little damage is actually a good thing - it wakes up the repair mechanisms and sets them to work, producing an overall net benefit. Thus a given form of damage may be good or bad for system longevity, depending on its degree, where it happens, and whether it is noticed by the repair mechanisms. This is why you'll see superficially contradictory research papers on reactive oxygen species, the damaging oxidant molecules emitted by mitochondria, and their impact on aging. See this, for example: "researchers have identified a pathway by which reactive oxygen species (ROS) molecules, which are usually implicated in the aging process due to their damage to DNA, can also act as cellular signaling molecules that extend lifespan. ... Increased ROS, and their effects at the cellular level, can lead to oxidative stress, which is involved in many diseases and aging. But ROS are also necessary for the proper functioning of the immune system and other biological functions. ... Inhibiting a signaling pathway called Target of Rapamycin (TOR), which is involved in sensing nutrients and cell growth, increases lifespan in yeast, as it does in mice. ... a key way this occurs is by altering the function of cellular powerhouses called mitochondria so that they produce more signaling ROS. ... The concept that ROS are important cellular signaling molecules, and not just agents of damage and stress, has grown to be widely accepted. Remarkably, in this study, we show that their purposeful production by mitochondria can even provide an adaptive signal that can delay aging. ... Trials targeting the TOR pathway as an anti-cancer strategy in humans are already underway. Our study suggests that carefully augmenting mitochondria and ROS production in humans may also be beneficial in combating aging and associated diseases." Note that "carefully augmenting mitochondria and ROS production" is a fair description of the results of exercise, and is one of the ways in which exercise works to improve long-term health. You may recall that researchers demonstrated that antioxidants applied generally tend to block this effect by mopping up the ROS that act as signals to the body's repair systems.

Link: http://opac.yale.edu/news/article.aspx?id=8625