Well, in regard to the second thing: Forum posts are dated. That acts as its own kind of copyright. So if someone does steal your idea (which, honestly, doesn't happen as often as people seem to act like it does), you will have posts about it dating to before they do, which will prove it was your idea first.
Also, if you're really concerned, write out everything you possibly can about your idea in detail, then physically mail it to yourself. Then DO NOT open it unless you need to use it as evidence in a coypright dispute. Post Offices date everything when it comes in, as well, and that date will hold up if you need to prove that you had the idea first. "Poor man's copyright", one of my teachers called it.
As to the first thing, a lot of people here are teenagers. Don't worry about it.