What is old to me? It depends on what we are talking about. If we are talking about pro fighters or athletes, 34 and above is old.
Actors are not the same as a pro athlete, but maybe 45 and above or at least 50, especially for beefcake actors. Arnold looked really good well into his 50's and actually looked much more handsome than he ever did when he was younger. When Sly made Cliffhanger, Demolition Man, and The Specialist he looked like a god and he must have been mid to late 40's. Of course, these are not average guys and probably no one should be compared to them.
I really liked the way Dolph looked early to mid 90's He was clearly aging and started losing a lot of his physique but he had a handsome, mature look about him. He also gave off the aura of a guy that had it all together and seemed like a real star, even if he had never achieved A-level success.
Keanu Reeves seems to have aged very little, at least not for the worst, and as far as I know he doesn't even take care of himself and is a smoker. I guess he is just blessed. He is about 50 and people find him sexy as hell.
I elaborated a lot and this post probably seems a bit gay, but oh well. It is part of my love and admiration for these guys.