Psychologists have studied this, and particularly a great british psychologist in the 19 sixties and seventies named raymond cattell who detected a certain kind of intelligence called fluid intelligence. It peaks around late thirties, early forties, and goes in a decline. Burn out in your forties is largely because of this decline. And so you'll find these weird things in highly skilled professions. You know, your dentist, who's a hot shot dentist around age forty three, just suddenly decides to take off fridays and golf. that's the reason that basically all diets fail. The same thing is true in our lives.