Most of the youth leaders in my youth group are college age, but a few are older. I don't think hardly any of them are older than thirty.
I don't think it's necessary for them to be younger, but I think that it can help - it makes it easier for everyone to relate to. E.G. one of the youth leaders might give a message, and if they give an example of what they're teaching about, then we have an easier time relating to it.