Well it's a combination of reasons, but it all comes down to most of them not having "brakes", figuratively speaking. Meaning that, when they're disrespectful, being arrogant, spamming a FPS game or guild Vent with obnoxious banter, going on about dumb stuff, trying to act tough, and etc etc, they don't know when to stop, let alone know whats right to say and whats not and when to say it and when not to. So us, older people, having deal with youngins in previous experiences, opt to make it easier for ourselves and write off any younger folk, unless they can prove they have a decent level of knowledge and wisdom.
Another important factor is that, being my age (25), it's hard to really relate to many 12-14 year olds on an intellectual and maturity level for the reasons listed above. But also, there is like an age range that most people, like myself feel comfortable socializing or talking to. In all honesty, does it not seem awkward if a 40 year old man and 12 year old were hanging out regularly? I know it seems like an extreme comparison, but I had to say it to prove a point.
And to be frank, once you 12-14 year olds grow up, you won't be able to bear through most 12-14 year olds conversations and company as well.
P.S. Oh and many of them tend to whine a lot, and beg for stuff on MMO's.