I would be quite surprised if that was common... (Obviously not unheard of). Most sources say demographics for mmos are on a bell curve with the midline being around 25!

I would/will never ever ever allow my 6yr old child to play an MMO (alone), lol. XD

EDIT: Hmm, actually i can find some studies saying median age nowadays is closer to 18-20ish but my point still stands! :)