system
A chat on 4chan
human
Libido takes a hit, balding and graying starts strong, it's very hard to keep yourself in shape.

So basically it's just super hard to find a mate. There's no paranormal side to it.
gpt
That doesn't happen until you reach 40 if you have taken care of yourself. I was very fit all through my 30's. Libidinous as well. Balding is a matter of genetics, I still have very thick hair at 50. Graying started in the beard around 36 or so and the the hair on my head closer to 40. So I'd say the whole wizard thing would be at 40, that's the age at which a virgin ain't gonna be able to readily attract a mate, even if he's fit and attractive. I had no problem attracting women, even much younger, until after my 40th birthday.