I really don't see this getting approved, but if it does happen to get approval, how would you go about regulating it? Make people use credit cards to verify age? Anyone can say that they are 18 or older when filling out the account information.
And if it comes down to click here to leave or 18+ click here to enter, well I think we all know where that will lead. Everyone who wants to enter, regardless of their age, will just click the 18+ to enter link.