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The Foglios are pretty fond of this trope in Girl Genius. In part this is because it's a World of Badass, and any serious contender who isn't dead yet is obviously not to be crossed.
Klaus Wulfenbach - the Big Bad(?) of the series - is actually a prematurely aged father, but has the two qualities of (a) looking elderly and (b) dominating every other mad scientist in Europe with an iron fist.
We'll mention Dr. Sun next, not just for his ability to make Bangladesh Dupree sit down and be good, or his willingness to tear apart a twenty-five foot mech with his bare hands, but for the expression of relief Klaus gets when their disagreement over the definition of bed rest does not end in a confrontation.
Ol' Man Death qualifies as this and as a (semi-)Retired Badass. The guy 'rode with the Jaegers' in his youth, is referred to by Maxim as the greatest warrior ever, and despite his advanced age can toss a Jaeger (Maxim again) through a window without breaking a sweat.
Remus gives us Marcus Laurent, a gruff, Two-Faced CIA agent who's part of Williams's grab squad.
From Schlock Mercenary is (retired) GeneralKarl Tagon, who took on a squad of mob enforcers in the buff with nothing but a show-piece carbine and inflicted heavy casualties before they managed to stick his severed head in a jar. And he still managed to escape.
Commander Heroic in Evil Inc. still fights crime when not in the hospital.
White Noise is one in Last Res0rt, who functions as the Captain of the Executioners (and thus, the other players on the show).
In The Inexplicable Adventures of Bob! Nemesites are an alien race with a lifespan of about 2,000 years. Fructose Riboflavin is 1,900, and looks it, being incredibly frail-looking and haggard. He can still outmaneuver and disable a hulking guard robot and make it look easy... and then complain that his back hurts, but that's okay.
Dragon Ball Multiverse: Krillin (or "Kulilin" as the author insists on calling him), of all people. He seems to have mellowed out in his old age.