[phlogiston]

Hrmm. I can see it may be confusing to have two seperate types of joinable group but to be honest, personally I'd still go with trying it via usergroups - vbulletin already deals with all the permissions for forum access by usergroup which is easier to use than hooking in with conditionals to use profile field based permissions (dealing with forumhome, forumdisplay, search results, forum jump menus, etc, etc)

Public Joinable groups are usually (I think) added as a secondary group, make sure when you create the usergroup to leave "Can override primary group title/markup" as unchecked, that way none of the settings or stuff from the group will show on profiles or postbits etc.

Also I may be misunderstanding - you say you'd have to verify them as over 18 for custom profile field method so I'm not certain how approving Join Requests would be more of a chore? :S