In my own experience I've certainly met kids like Fraid who can behave just as well as (if not better than) most of the adults I've encountered in WoW. I've also met players who came across as bragging and became very difficult for me to deal with until a friend said "They're only 12," and I understood. After that, I found it much easier to tolerate the things they were saying as just part of being a teenager rather than some plot to make me feel inadequate. Another time, I went through several 2-vs-2 arena matches in which my friend was strangely quiet, and yet playing very well. Afterwards, my friend explained that it wasn't her at the keyboard, it was her 10-year old friend. This friend isn't very good at typing and talking, but he certainly knows how to play!
Kids can be fantastic players, or lousy ones, just like anyone else you might encounter -- it isn't right to judge them based on age alone. The fair way to deal with kids in WoW is to give them a chance to show their skill, and even to help them improve however you can. If they prove unreasonable, then kick them out the same way you would an unreasonable adult. For those adults who just exclude young people from their activities altogether without giving them a chance, there's no denying it is prejudice -- judging before knowing -- and it often means depriving yourself of some quality talent for your group. Of course, if your guild does some sort of ERP, or other "adult" activities, then by all means, exclude away! But in normal gaming, especially in a casual guild or a PuG, there's really no reason a kid shouldn't be allowed to join unless they've already shown themselves unable to do what's required of them. Anyone who says, "some of these people are not fun to be around, so I'll just exclude the whole lot of them," has some growing up of their own to do.