One of my favorite WoW podcasts(besides our very own, of course) is Casually Hardcore from WoW Radio. They're a very fun bunch and they cover the topic of age fairly often in their fan mail.
I realize a lot of young people play WoW, but I haven't had much direct experience with the really young crowd. As far as I know, I've only played with someone under 16 a couple of times since late 2004. I'm 21 now, and I'm actually the third youngest person in my guild, and there's only a few others younger than I am in our entire guild alliance. We probably just fall short of 200 people across all of the guilds, so the idea of people so young playing WoW is actually quite strange to me. I see parents saying "AFK, my daughter needs me" more often than kids or teens saying "sorry guys, I have school in the morning."
While there are definitely mature younger people playing WoW(I like to think that I was/am one... maybe) but I haven't honestly had much experience with the younger crowd at all. I'm willing to bet money that my guild's average age is somewhere in the high 20s. What about you guys, willing to take a guess? Is your guild younger? Older? A mix of all sorts?