You said it yourself.
"a large proportion of 12-14 year olds are annoying imature idiots"
People will never stop generalising. It is the only way they can try to prevent themselves from interacting with bad parts of anything. It is ultimately futile, because by now the only ones who actually admit that they are 12 are the ones who are actually good - the bads know that they are bad and try to hide all signs.