It really depends on whether or not you regulate his time on it to make sure he doesn't become TOO addicted. But don't be like, "You only have one hour per day," because that's just brutal, and if he's in the middle of a mission, I recommend letting him finish it (unless it's gonna take a looooooong time) for the sake of fairness to the gamer. However, as I said, regulate it. Be flexible, and be fair, but regulate it so that it's not the only thing he does. As someone said, it could be his social outlet, and this is something that's very very important to people!
I haven't played the game itself because it's an MMORPG with monthly fees, and I'm not financially stable enough to handle those monthly fees, so I guess I'm not really the most...suitible person to be giving advice about this. But I wouldn't consider it to be a matter of age; WoW is, after all, rated T for Teen--13 and up (though in today's world it's more realistically 10 and up), so in terms of violence and what not it's not bad, generally appropriate.
---In response to the other games---
There's PLENTY of games out there! If you want a good gaming resource site, you can go to http://www.gamespot.com/
If your son's into MMORPG's (Massively multiplayer online RPG's), than he can try similar games like Guild Wars, which is the only half-decent MMO with no monthly fees. He can also try just playin' old RPG's (role playing games) like ones from The Elder Scrolls series (Morrowind, and more recently Oblivion--which I only recommend if you have a KILLER system), or Star Wars: Knights of the Old Republic, NeverWinter Nights, or the more mature ones like Arcanum, [b]Vampire: The Masquerade - Bloodlines, and Planescape: Torment (these ones are quite Mature-Rated, however, with more profanity and gore and what not). If he'd like to try different genres he could try first person shooters (though most of these are M rated and quite graphic), platformers (like Psychonauts and the Prince of Persia series), Real-Time Strategy games (such as Warcraft III (which has the same universe as WoW), or Stacraft (universially regarded as one of the best games ever made) ), "sneakers" (the Thief series and the Splinter Cell series), or racing games (Need for Speed series, etc.). I also recommend The Sims or The Sims 2--they can be quite addicting, however, but almost any decent game is. ^.~