Because kids are small and anyone who is bigger

than them, simply "must be a bully". Not my rationale, but most peoples'. Another thing is that most people think a kid has to complete schooling (graduate high school) before they have adequate knowledge (and it sucks that beurocrats get to decide for us all "what is adequate") to make wise choices especially regarding sex and marriage. And any adult who would consider being with such inconplete human beings (as those who haven't grown to as tall as their gonna be or who haven't complete their government programming / graduated high school) can only be interested in taking advantage of such "poor defenseless" people. But ain't it funny how when those poor defenseless ones (the under 18-ers) commit a high crime, and they get tried and sentence as an adult.
Oh the contradictions.
If the beurocrats were consistent and truly worthy of respect, they'd say this:
"Yes, this kid has commited grizzly murders. But his prgoraming is not yet complete, since he has not yet finished high school. And for Christs' sake, this child has not yet received the wisdom that miraculously flows into every one alike, the morning they wake up an 18 year old."

Well Ominous. I hope that's a little help for you.