The most likely culprit, IMO is the anti-cancer function of the immune system. You take cell damage at a relatively predictable rate (at least, on the low end)

The most likely culprit, IMO is the anti-cancer function of the immune system. You take cell damage at a relatively predictable rate (at least, on the low end), and once enough damaged cells have been destroyed by the immune system you start to run out in vital areas.