I wouldn't call it an age but more like a stage. A child like mine was definitely curious about my husbands crotch looking different in his underwear when she was about 3. He felt that was the age to stop being where his bulge could be seen by her.

My mom said by the time i could talk she felt it wasnt a good idea to see my dad naked. He wasnt comfortable with it by that point either.

My mom seperated her male/female grandchildren from the bath when one of them started to be way too preoccupied with the other ones body parts.