I don't think that's necessary at all. We don't need to introduce any segregation of players (and even equipment) here. The only thing that's necessary is to do what common sense (and the law) demands: prevent underage players from entering any credits/prizes transactions. This can be easily arranged in the database by adjusting user permissions. WGT simply need to set up another category of WGT.com user accounts – one which will be by default prevented from entering any credits/prizes transactions.
I manage phpBB forums, and whenever you want to join a phpBB forum, the standard question asked when you're signing up, is: Are you over 13 years old? If the user answers No, then there are (by default) certain additional requirements to be met before the phpBB account is activated.
WGT can proceed similarly: during the sign-up process, they can ask the user, Are you over 18 years old? If the user answers No, then the user account created during the sign-up process will not be allowed to enter any credits/prizes transactions, but will otherwise be able to enjoy WGT.com fully with all its other features.
Issue solved!
(A parent or any other adult well-wisher might still gift equipment to an underage player using the standard gifting procedure.)