User blog comment:NecrusIV/Average age of a Space Marine/@comment-70.192.195.53-20140901144427

Dunno if this is still a topic of debate, but fanon wise I'd keep it, on average, around 300 for a veteran commander. Absolutely no more than 500 for a Chapter Master(rare to make it that far also). That's what I think a good limit should be.

Any debate regarding "what age is too old" for a Marine to still be able to fight is rather mute, as loyalist marines have no canon expiration date and ALL die in combat eventually(or for the extremely rare occasion, get placed in a dread and put in a museum *cough Bjorn cough*).

The debate should be "how long on average until a Marine dies in combat".