Well, at least it isn't on 99chan anyways. I know it is on 4chan sometimes, I know of someone who was banned from there /b/ board for it (and consequently fashion, which was the main board they posted on). Other than that I don't know of anyone who's ever been banned for it. I can't stand when someone posts, even if they're 16 or 17, and then some moron out of the blue goes HEHEHHGHHURRRRR UNDERAGE B&!
A good percentage of posters are probably underaged anyways. Maybe that's part of why people like to call them out on it. Is it fun to try and identify underage posters?
In the end it doesn't really matter if they're underage or not, as long as the contribution to the conversation isn't something fucking retarded. I guess part of the fear that people might have is that if the userbase becomes younger the conversation might start to loose quality. People who fit into this category seem to range from 14 or younger up to 17.
We can see this on websites like Rotteneggs.com, posts become mostly about angsty things like anarchy, bomb making, or things you could set on fire.
Where did underage b& ever come from anyways? Must have been the shown age requirement on some of the 4chan boards.