IM, I suggested this once - slightly different reasons though! But I had it quite rightly pointed out to me that any kind of marker would also be a magnet for people who had less than honourable reasons for wanting to know who the minors are. I didn't think of it that way until it was pointed out, but its obvious really.
I think it is down to parents to supervise their kids anywhere on the net.
I would suggest that maybe you shouldn't flirt until you've talked to a person for a while? If they are pretending to be older than they actually are, it should become clear after a bit of conversation.