>>28278870/b/ and /pol/ are plagued with underaged desperately trying to fit in. The kids just want to post on those boards to say "nigger" and have edgy opinions and not get punished for it.
>/co/mblr'Nuff said.
/v/ attracts probably the most underaged. Gamer culture is super trendy and the types on there love to be contrarians. Gotta be too cool, cooler than casuals. We get some of those on here.
This is all just off the top of my head. Granted, those types have overlap with your usual autistic manchild, but not every normalfag is the "lol XD" type. Many post normally, but just don't get the board culture.