Thank you for being a nice user. I am under my parent's permission.
To be honest you can do whatever you want on the internet.
I don't think I've ever seen anyone arrested because there was a kid on their forum/they registered on a forum as a kid.
Actually, a few big name corperations have been sued over the matter:
The FTC has brought a number of actions against website operators for failure to comply with COPPA requirements, including actions against Girl's Life, Inc.,[4] American Pop Corn Company,[5] Lisa Frank, Inc.,[6] and Mrs. Field's Cookies and Hershey Foods.[7] In September 2006, the FTC levied substantial fines on several enterprises for COPPA violations. The website Xanga was fined USD $1 million for COPPA violations, for repeatedly allowing children under 13 to sign up for the service without getting their parent's consent.[8] Similarly, UMG Recordings, Inc. was fined USD $400,000 for COPPA violations in connection with a Web site that promoted the then 13-year-old pop star "Lil' Romeo," and hosted child-oriented games and activities, and Bonzi Software, which offered downloads of an animated figure "BonziBuddy" that provided shopping advice, jokes, and trivia was fined USD $75,000 for COPPA violations.[9]
But if you add in a "you can't be here unless your 18" thing for new users, and then never care about it or enforce it, you're pretty much exempt.