My guild has a few young boys, some of whom were twelve (or close to it) when I met them, and they're really nice people. Rambunctious with excessive amounts of time on their hands, yes. But still nice.
And I think it's a misnomer to call immature people 12-year-old boys like we do, because most of the really horrible people I've met (the ones who are mean for the sheer pleasure of being horrible) seem to be college-aged males.
That doesn't mean I haven't met 12-year-olds with psycho senses of entitlement. But it does mean that bad 12-year-olds are easy to spot, and most of the worst and most offensive players aren't kids. Bad kids tend to stick to "I deserve everything." Bad adults tend to revert that: "You deserve nothing."
So, yeah. If I'm stuck with a PuG, I'd rather have a selfish kid over a jerk adult any day. Like in the show Survivor -- a quantity you can predict is better to keep around than a quantity you can't. A kid who wants everything no matter what is better than a jerk who will sabotage himself just to sabatoge you.
What to Expect From a Kid Applicant
Kids have more time on their hands and will be bored (and thus harder to please) in a slow guild. If you have a guild that bustles, a good kid will improve your ability to provide groups for your most active players.
Good kids respond favorably to kindness. You don't have to be their mentor or BFF, just be (genuinely) happy to see them. I truly love seeing my guild kids because they're sweethearts, even though I don't run much (anything?) with them. Just treat them like people. Because they are.
If you can get a team of kids -- 5 to 7 -- with similar amounts of free time and interest in running things, they could very well become a self-sustaining entity within the guild as well as a great supplement to your slower players. They keep each other from getting bored and having to beg slower players to run something (or *shudder* pugging).
Weeding Out the Bad
Make sure everyone knows your rules going in and accepts them. Have a conversation spelling out anything you think might be a problem. (Guild bank rules are always helpful to detail, whether you're worried about ninjaing or not, as well as loot rules.)
Take a high-level kid along for a few raids (we've been leading 25-man pugs to Naxx every Sunday and got a few new members off of it). If the kid leaves the raid after getting loot or makes a fuss about not getting loot or insults the guild's proficiency, dump him.
Find out about his monetary situation and how he expects to improve it. Both our 12-year-olds who got kicked were weird about gold in particular. One planned to beg us for gold for his first mount (and told me so) and the other stole items from the guild bank to buy epic flying.
If he's leveling, find out how much help he expects. The first of our kicked kids expected something akin to an 80 escort whenever he wanted it.
At the end of the day, 12-year-olds get a bad rap. Sometimes they deserve it, sometimes they don't. It's true that their needs are different due to lifestyle differences, but other than that they are just like you and me. Just as reasonable, just as polite, just as generous.