To me it's about mentality, not about the years someone has been alive. I've already learned years ago that how long you are alive has little to do with how mature you are. And since then, when on the net, I've always been basing my age judgements on mentality. Heck, there's little else you can judge someone else on on the net with. Anonymity and all that.
Anyone trash-talking and insulting others like that has the mentality of a 12 year old to me. Why 12? Don't know exactly. But my guess is that that age comes closest to what people are reminded of when experiencing such people online. As in, old enough to have learned some 'new words', combined with hitting puberty and the inability to understand the connotations of said new words. As, can you really say that someone insulting your parents/sexuality so casually, has any real idea of what the words he uses entails? If he did, he wouldn't be using them so casually.