18+ may only enter

[Necron]

wouldn't usergroups be a good option so you can completely hide the adult section from u18's. if they saw it but couldnt get in, common sense would be to change the DOB to make you old enough.

if not, you would open forumdisplay.php and showthread.php, find

if (!($forumperms & CANVIEW))
{
print_no_permission();
}

and replace with something like-

if (!($forumperms & CANVIEW) OR ([ variable for DOB here ] AND $forumid == [adult forum ID here]))
{
print_no_permission();
}