Board Thread:Warhammer 40k General Discussion/@comment-24383134-20140105071847/@comment-4851119-20140105123758

Dreadnoughts provide insane longevity for their pilots depite the fact that said pilots were on the brink of death before being interred. Bjorn, the oldest living Space Marine who fought alongside Russ and The Emperor in the Great Crusade and is still alive and kicking in the 41st Millenium, is confined to a Dreadnought.

Dante is firm evidence that Space Marines don't age the way normal humans do, seeing as he's had no loss of ability or anything for biogical reasons despite being around 1300 years old. It's most likely because their healing factor cancels out their ageing process by healing the degradation of the gene that causes ageing in humans, though that's just speculation on my part.

Whatever the cause of it is, The Gildar Rift, an official canon novel, contains a line from an Apothecary stating that Space Marines are functionally immortal outside of a battlefield. I'll take that as true any day.