I want to know why you want a teen board when this was all sparked off by a few posts by a couple of pre-teens.
I'm all for a children's board, correctly monitored, but I think you'll find that teens (at least the ones I know on here) are quite happy posting on normal boards.
As grenv said, people can be over-protective and brainking is one of the best sites for moderating.
Undesirables can be banished at the click of a mouse.