i think it depends on the person....confidence i think matters..
my grandfather is about 70 now and he is still having an affair with a 23 year old girl...as far as I can see from the girls face if I see her, she looks satisfied....
my mother said, my father quit (i'm not if it is the right word) at 50....
i guess it's because my grandfather thinks he is still at his age while my father thinks he is too old...
i'm not about it though....