Haven't played online gaming since the launch of Halo 3.
The amount of homophobic/racists insults hurled at me (despite being a straight, white guy), by incredibly squeaky-voiced people, just because I'd killed them (the entire point of the game) was ridiculous. And this was every single time I'd played. Put me right off.
The reasons most 13-year-olds are looked down on in gaming are for similar experiences above. Plus the fact they're 15/M/18 rated games which shouldn't be played by 13-year-olds anyway, at the least not online.
But if they(/you?) keep quiet, are polite, and limit mic usage then most people won't even realise you're 13. Just break the stereotype, and you won't be stereotyped by people online. But the stereotype exists for a reason.
Much prefer gaming with 4 buddies in the same room. Much more entertaining (or, if you have space, linked Xboxes with 2x teams of 4 in different rooms). Not playing online isn't too detrimental to me as I don't own a 360/PS3 (was playing on a mates whilst he was on holiday - he said I could).