Searching in the Diablo Wiki I happened to stumble upon this timeline of Sanctuary's history.
When we first encounter Deckard Cain, in Tristram, he's already an old man. He claims to be the last of the Horadrim (even if this seems to be disproved in Diablo III), which seems unlikely, because more than 150 years have passed since the last known activities of the order. Anyway, in Diablo III, it will be more than 20 years since we first met him.
So my question is: what's his real age? Does he have a dark secret that keeps him alive? He managed to survive events that killed many other younger men, is he just lucky?