Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150723053637/@comment-6078851-20150724154624

astartes are often referred to as 'effectively immortal', but this isn't strictly true. they have incredible longevity - living as long as a thousand years and more and can heal from insane physical damage, but as mentioned they do age. ageing is simply the degredation of cell-reproduction, and such degredation will eventually reach a point when cell-reproduction is no longer viable, and therefore death will follow.

the problem with astartes though, is that none have ever died of old age, so until an astartes does actually die of old age (if ever) then the term 'effectively immortal', while not true, has a degree of validity.