When do we hit middle age? I think most people would place it at 35-40, but I see people who look middle aged at 33 and some who look youthful at 43. It's probably not a matter of one's chronological age, as much as it is the arrival of certain characteristics. Maybe when the body starts to break down. Maybe when you don't want to play basketball and football any more. Maybe when you get a bald spot.
Or maybe, to borrow from George Scott-Moncrieff, it's when "one realizes how short, after all, is life itself, and one's attention travels to the narrow gateway."