That IP bans kids under 13 for the remainder of the time until they turn 13 automatically if they join with their real birth date.

If they have parental content, they can still be here. Also, some are actually mature unlike a certain topic creator (no means for flaming, just stating the obvious)

Well, if they have parental consent they can be on forums legally, but our rules can be changed (if they don't already) to just plain "No one under 13".
Though I agree that their are some underage that are mature.
Besides that, the majority of underage forum joiners will say they are older than they are.
I still like Hakker's idea of mental age determined by their actions, but it's been turned down several times already.

So as a general statement, lets leave underage people alone, unless their actions show that they are so obviously underage and immature that they don't belong here.