Interesting article from Alasdair Wilkins on io9.com:
All animals eventually grow old and die. It’s an inevitable fact of life — except when it isn’t. Some animals, like tortoises and lobsters, never grow old, and learning their secrets could let humans live as long as they want.
For most animals, there are three basic ways they can die: disease, injury, or old age, which is also called senescence. But a select few species are seemingly immune from aging itself, a phenomenon known as negligible senescence. The gradual accumulation of cellular damage and degradation that will eventually kill other animals (including us) slows to a virtual standstill, prolonging the life — and, in fact, the youth — of any animal lucky enough to be negligibly senescent.
Tortoises are the most famous negligibly senescent animals. An Aldabra giant tortoise named Adwaita was thought to be 255 years old when he died in 2006, and carbon dating of his shell confirmed that he really had been born around 1750.