No Space marine has ever died of old age... they get too old.. 500+ years and then one slip up and poof dead... saints were tourtured till They lied and said they were heritics or diceted till they died... Fates fare worse for those that clame to be deamons.
Well before the horus hereicy there ther the Protomarines that fought on earth on holy terra while the Emprior rised to power. one such proto marine was kept next to the warmaster Horus him self because he was the "father" figure of the legion... His body was made diffrently and he had the blood of the the Empiror him self in him VS having the blood of the Primarchs. He aged and was close to 1000 years old and was eh just a rambling old man... Like some old brittish wolrd war 1 vetrian rambling on about how the war was like while ww2 was going on
The funnyest thing about warhammer is it was made in england so if you read a few of the books it has dry brittish humor...
One story I read was about the space wolves and Ragnar and one of his brothers had a stolen heave bolter.. they were pinned down by NME (say it slowly I can't spell that bleeing word) so his brother hooked his arm around the corner with the heave bolter and fired till the pin welded it's self to the barrle... he looked at it then tossed it around the corner.. a second later they hear a skull cracking sound and the whole squad looks at their brother while Ragnar sais "I think he invented a new way to fight... throw the heavy weapons at them"
Another funny thing is if you ever play dawn of war and have a squat under a drop pod... the fly up into the air land on the ground then look about wondering where in heck did the pod crom from and then move on as if nothing happened..