A mix of settled life, taking cities at least every 15-20 turns, and MASSACRING said cities.

I'm serious. I have some freakishly old family members; 6/8 are over 75 (thank goodness there 3 sons and 2 daughters almost of age...that was a close one), at least 2 are over 80, and those two both have that "massacre" trait. Had the same thing with an old Rome campaign where my faction leader lived to be 99, had topped out command, influence, and management, and the massacre trait. The game had to be ironic though, and make his son kinda stupid, with the "likes to play the submissive role for other men" trait...bwahahahha! I couldn't believe it when I saw that.

Maybe its all a coincidence, but maybe making your members bloodthirsty, Chingiz Khan types does something to their longevity?