Men like to fuck attractive females, women that they consider "hot". It just so happens that women look their best when they are in their late teens and early 20's(with rare exception).
Do they really? I'm not even making a value judgement here. Personally, I have always found women most attractive around 30. I prefer women who are past that bubbly stage. A good deal of the women I went to high school and college with did not peak btwn 16-21. I would say more than not got progressively more attractive up until about 30.
I think there are women who peak physically around 20, I just feel like it's not really the standard. I think guys who are going after "barely legals"

might be attracted to other things, like a greater number of prime years or uncomplicated relationships.