It's common knowledge. And that happened not just on the middle ages.
Interview with former Brasil president Luiz Inacio Lula da Silva in 1979:
Playboy - With what age you've had your first sexual experience?
Lula - With 16 years.
Playboy - Was it with a man or with a woman?
Lula - (Surprised) - With a woman, of course! But, in that time, dirty stuff was much bigger than today. A boy, in that time, with 10, 12 years, already had sexual experience with animals... We did a lot more nasty than the kids do today. It was a more free world...