The same is true for intellectual work. An eight year old can work quite hard to solve a math problem, the same math problem that a 35 year old will solve without making much of an effort.
As for the difference between men and women, we generally say that women are better "multitaskers" than men. This is actually not true, here's one of several studies showing men are actually better multitaskers, but this myth that women are better multitaskers comes from the fact that men are also better at focusing (working harder) to solve an intellectual problem. So yes, men do work harder (on average) than women, even with intellectual work. That's the reason successful societies developed by letting men do all the hard and specialized work. "Equality" will make our society far less efficient, which means societies less concerned with equality will overtake us.
Yes, I know, I'm misogynistic for saying this truth.