[Lynne]

You want to create a new usergroup for the Over 18 bunch and under Public Group Settings, set "Public (Joinable) Custom Usergroups" to Yes. When you create the new group, on the top of the form set "Create Usergroup Based off of Usergroup:" to Registered Users (usually that is best) and hit Go and it will put all the same usergroup permissions in the form, then just change the name to "Over 18 Group" (or whatever). Then in the Forum Permissions page, give this group access to the Over 18 area.

If you want mods to approve users to the group, then on the Usergroup Manager (after you have created the group), find the group on the bottom of the page and click Add Usergroup Leader and input the name of the mod you wish to approve the users (you may add more than one). I think this group leader also needs to have Admin Permissions - Can Administer User Permissions - Yes.