After you install the User Groups mod, create a user group called "Minors" and configure it to deny access to any "Adult" area of the board.
Create another user group called "Members" and configure it to allow access to any "Adult" area of the board.
When new users register, anyone between your minimum age for registration (default is 13) and the age of 21, will automatically get assigned to that "Minors" group.
When the minors turn 21, they are automatically moved from the minors' usergroup to the members' usergroup and allowed access to those formerly restricted areas.
Get it at SnitzBitz.<

Why not make the age configurable for countries that have a more normal view of what age someone is no longer considered under age?
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It is configurable. It's a small file, download it and read.<