It is frequently asserted that Astartes are functionally immortal, and only die in battle. But this doesn't ring true to me. Some of the background material doesn't match up.
Sergeant Telion of the Ultramarines is obviously old. As in, he's clearly gone bald and his mustache and beard are white. The Space Wolf Long Fangs, among presumably many other examples, now have grey hair and beards. Obviously these are signs of aging - if Astartes were functionally immortal, why would they age?
Logan Grimnar is about 700 years old. His mentor Ulfrik the Slayer is apparently even older, with white hair. These are probably the uppermost limits of what Astartes can hope to reach in terms of age. Meanwhile Chaplain Cassius is the oldest serving Ultramarine, weighing in at nearly 400 years; maybe Ultras are just shorter lived or Cassius is the only one in the Chapter badass enough to live that long. Or - duh, stupid me - the recent conflicts with the Tyranids killed off most of the older guys, leaving only poor Cassius as the eldest.
Dreadnought pilots evidently become more and more senile over the years. The old White Dwarf article has this to say: "When not in action, Dreadnoughts slumber within sealed stasis vaults in the depths of their Chapter's fortress monastery to extend their lives." The mental deterioration points to, well, aging.
Blood Angels are noted for having unusually long life spans: "Physically the Blood Angels are among the longest lived of all the Space Marine Chapters. One of the peculiarities of their gene-seed is that it has vastly increased the lifespan of those who possess it, so it is not uncommon for Blood Angels to reach a thousand years of age."
So, say that an Ultramarine and a Blood Angel have roughly the same fighting skill, they are both badasses and no one can kill them. The Blood Angel could live up to a thousand years or more, but the Ultramarine... dies? The Blood Angels are specifically noted as having a lifespan, so at what point might an Astartes die of natural causes?
There are repeated mentions of older Astartes that become too injured to fight, so they become senior officers, trainers and instructors, and possibly other functions as well. This is the real reason why I ask this question. What happens to these Astartes? Do they grow old and die? To me that seems like a pretty bad way to go, since Astartes wish to die in battle; it is the nature of their existence after all. Do these Astartes eventually commit suicide, so they don't have to grow old? Do they, after serving the Chapter for long enough, get the opportunity to strap themselves onto missiles and fire themselves at the enemy for one last hurrah?
1/16/12 One small addendum. Imperial Armor Vol. 9 makes mention of Stilbor Lazaerek, Grandmaster of the Fire Hawks during the Badab War in a sidebar. Lazaerek is described as being over 600 years old and in "failing health" because of wounds sustained over his long career. Interesting.
