In his memoir, Making It So, Patrick Stewart (Jean-Luc Picard, Professor X, etc.) said that he was noticeably balding at 17 and was more or less completely bald by 19. Stewart may be an extreme case but a sizeable population of men lose a substantial amount of their hair before 26.
yeah it's kinda weird that we associate baldness with age, most of the people that get it genetically seem to start balding at a young age. I have a friend that started losing it in his teens and had lost enough to constantly wear hats by college.
I've met a few people who had a clearly visible bald spot at that age (including my father, fortunately I got my hair from my mother). Not many, but it happens.
