Well, I believe that the underage look appeals to every last male in some form or another. Narrow hips, shaven puss, schoolgirl outfits and tiny tits.
Everybody started out wanting something.. or rather.. 'someone' original. Someone untouched. Virginal. Then reality hits, nobody is a virgin, half the girls are pregnant or married. (granted, 3/3 of my last gf's were married.. a'course I only date MILF's.. but.. )
Anyway, the marketing in America doesnt help, either. Underage singers, pubescent-looking actresses.. advertising with not-so-subtle pedophilia written all over it.
Anyway, all biases and stigma aside.. its human nature to want the youngest, most attractive things. If it wasnt for the law, then 'if they're old enough to bleed, they're old enough to breed' would still be popular. It was in ages past. Brides at 13. Mothers at 15. Etc, etc.
P.S. I know I'm certainly not alone on this, but my own fetishes hint me towards the 'motherly' types. MILF's, and as my avatar suggests.. a rounder, softer.. more full-bodied Jennifer Tilly, Anna Nicole Smith type... but the hard on is fickle. Luckily bearshare and bittorrent are always available.