Once again, answering an old question in case someone else stumbles on it. :-P
You should know though that any kid I’ve ever played with knows how to uncheck the language filter. There is no parental control other than a checked box. Kids will be kids and they will want to say SHIT instead of @#$%.
The game itself is teen. The gamePLAY is not rated as it is online content and cannot be. It ranged from kid talk to R. But generally people are reported for bad behavior, same as on Fluther.
You should know that if this kid wants the game you’re going to need to buy him World of Warcraft, World of Warcraft: Burning Crusade, and World of Warcraft: Wrath of the Lich King for him to enjoy the fll experience. Buying in prepackaged sets will be cheaper overall.