Firstly, I 100% agree with what Allegory said. I've seen 10 year olds more mature than 30 year olds.
I'm only 15 - I've been punted out of groups because I'm '2 yung 4 dem and wil wind dem up'. It got me so wound up, I just logged off and resorted to a bit of counterstrike. (We had an 8(EIGHT) year old play in a clan match last night - he came in second, over a couple in their mid twenties, age seriously does not matter. Maybe bedtimes, however, but he was on untill 11pm, which made my stomach turn a bit)
I also agree with those squeaky people on Vent/TS, albeit I'm one of them (My voice hasn't broken yet, goddamn it better break soon, or I'll break it myself). I'm constantly being asked if I'm a girl - I'm fine with it, I wind them up by saying yes, waiting for their reaction. (Some people think they can go further than asying 'ok' ^_^ <3 those kinda people so much!). Because of this - I hardly talk, I might say 'Hi' or 'G'night' when someone joins or logs for the night, but that's about it. (Although I sometimes join with a friend, who's a year older than me to talk, he denies I sound like a girl, but too many other people think I do)
I think people have to stop slacking young people off. Sure, there are lots of kids that just go around jumping around ninja-pulling the whole goddamn instance. That's why you can kick people out of instances; ignore them; and if they're swearing at you, deliberatly winding you up - generally griefing you, just report them. I was reported for swearing at someone who was supporting Gold Buying. IN. THE. GUILD. (Not saying swearing's allowed, I meant the RMT thing.)
Agreed, age plays a fair share of it, but it's their maturity that finalises it. There's too many other, maybe smaller factors in it just to blanket 'Youngsters'