User blog comment:NecrusIV/Average age of a Space Marine/@comment-1583177-20120817203428/@comment-1583177-20120817220617

The gods have their champions, and several individuals (like abaddon, Karn, Lucius, Ahariman and many besides) who they have poured a lot of time and effort into molding them into their champions, it would be simple for them to mess with fates and the future to keep them alive in regards to luck, or to mutate them and give them immortality (like lucius can become anybody who kills him etc). Also look at the life sustaining technologies page. It says that warp energy does extend life at the cost of spiritual and physical corruption (unless your the emperor, who is in a different category).