I'm not sure how to answer that one.

Whatever you do for this situation, though, record all the steps. That way you'll know what to do if/when this happens again.

If it were me, I would contact the person say that because of a certain revelation on your forums, you need to take extra steps to protect yourself and your users. I would then ask them to fax me legal proof of their age (driver's license, passport, or birth certificate). If they were indeed under the age of 13, I would terminate their account and not refund any monies (that last part depends on your own TOS and the laws of your locality. Consumers in some states are guaranteed refunds of any prepaid unused product or service, regardless of any contracts signed with providers.) If their legal proof showed them as being over the age of 13, I'd file it. If this person, or this person's parents, then tried to sue me and / or my company because they were underage and got into trouble, and it was therefore somehow my fault, I'd pull out that legal proof of age.

But that's just what I would do, based on what (admittedly) little I know of the situation. As always, IANAL...