User blog comment:NecrusIV/Average age of a Space Marine/@comment-70.192.195.53-20140901144427/@comment-8395098-20140902015435

GW hasn't officially set a age limit on the Spacemarine, and thanks to some of their staff wanting to have the greatest sues in existance the fact that Dante is the "oldest known living loyalist" is a moot point as there are several unknown or simply unpublicised SM who are apparently older.

However that said this is the 40K universe, there are still ships poping up in corners of the galaxy who think they are still scouting for the emperor's great crusade, not realizing that the warp has fucked with them.

however i'd say that Dante's degeneration gives a good estimate of how long the "average" SM can physically last if he is talented enough to survive past the given rough 150~200 year marker in which few marines seem capable of living past. (from their time relevence, warp time fuckery again)

and about the arguements down below, it's as stated the warp fucks with time you go faster than the room your in, or you age at 1 one millionth the speed of the man next to you. but also chaos is capable of creating new CSM so it's just as likely that while the champions have both been sustained by the warp and took good advantage of time fuckery there are just as many if not far more CSM who were made just as recently as the Loyalists they are fighting.