Because we're hormone exploding, impulsive, selfish, demanding and unreasonable pricks. Just like all people, frankly, some are worse and some are better, but in the end we're all just human.
Maybe that's a too negative view for you, but frankly, I'd rather face the truth and punch it in the face than stay in denial for the fact that no one is perfect and when you're not fully grown and stuffed with hormones, then you're going to be more or less a douche in someone's eyes. Although, you don't even need to be a youth. Everything's offensive to someone, somewhere, somehow, remember? Doesn't matter who you are, in someone's eyes you're a douche.
In general, this is just stereotyping, and the youth that makes the biggest mess gets the most attention, some people just don't have another view to go by. -Shrug-