Isn't that always the way though, the minority of people ruin it for lots of others I know 12 and 13 year olds, that would be responsible in the parks on their own. I think 14 is too old personally.
I must admit, though I've seen terrible behaviour at Disney, it's not been from kids on their own. In fact I'd say I've seen much less of that than in normal life.
Generally the bad behaviour comes from parents telling their brats to go to the front of the queue and call them forward and the like. The rest of the bad behaviour tends to come from the adults
I'm not the most kiddie tolerant I know, but when I HAVE seen kids on their own, they've seemed to take it pretty seriously IMHO for example I saw a young girl, 12 or 13 I'd guess, guiding two younger kids into the town hall at the magic kingdom and she was doing a much better job of controlling them than the adults who were there with kids