Of course 60 years is no physiological boundary. Still I think it makes a lot of sense: I would assume that the older you get, the bigger the risk of getting into a situation where your health doesn't permit a safe conduct of flying, even if you passed all your medicals before (no medical examiner can ever be sure to find out if a candidate will suffer from a trombosis, heart attack or some sorts of cancer, even the day after his examination).
Taking this into account it makes sense to limit professional pilots age, which should be ideally lower than for some other professions, which are normally 65 years.