Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150723053637/@comment-8464494-20150728035132

Astartes are a bit like Lobsters, excluding injury or temporal effects such as entropy fields, they could likely live practically forever. While they can certainly age, most Space Marines do not become really "infirm", a nine thousand year old SM is likely still more than a match for a thousand men if kitted out in his Power Armor. He may be a bit slower on the draw, but thats like saying and old gunslinger is only slightly slower on the draw, he's still got a helluva lot of experience killing things that would make normal men curl up and cry at the sight of. As for the actual upper limit of age, if you think about it, it may depend on the Chapter. Blood Angels almost never show signs of age, in spite of the fact that Dante is the oldest dude outside of a Dreadnought, so the degredation argument may not be valid at all. And consider that ever Chaos Marines from the Old Legions who still spend most of their time in realspace, like the Alpha Leigon and Night Lords, those dudes still hang around and some of them are GC vets, so they cannot become too feeble with age, lest those individuals die at the hands of their barbaric fellows. All in all, Space Marines can effectively said to be immortal because the point where they will die of old age naturally is so long and difficult to really guess that you might as well say they are immortal for expeidency.

That all being said, no Space Marine, Chaos or otherwise is likely to die of old age due to the nature of the lives they lead. Loyalists are always in the line of combat because the Imperium demands it, and Chaos Marines and Renegades are pretty much either always on the run or dealing with internal conflict, therefore they all gotta die at some point, its just the odds of when and by what.