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Like >>22738 said, Fury uses something called the Infinity Formula to stop his aging. Sliding timescale aside, its still 2012 in the marvel universe and Fury is still explicitly a WWII soldier. Unlike, say, Iron Man who keeps getting the war he started in updated. I don't know if Dum Dum uses the Infinity Formula, although one would assume he has access. I don't know what, if any, explanation they have for Bucky Barnes not being 80 something.