There is no way to prove if you are a boy or a girl. And there is no way to contact others.
If you don’t like it, find another site that is better. You won’t. There is no place as kosher as YWN on the internet that allows user interaction and is as safe and well-run.
Also, what does age have to do with it? If you think that it’s bad just because it’s high school girls, let’s say you were talking to 30 year old married women, halachically that would only be worse.