Inside groups you can set several join types.

Invite, Open, Closed and request only (others available)

I suggest adding either an Additional join methored or a group setting applicable by the group owner Only (or however seen fit).

Its job to allow the group say to be open to users that have the 18+ Tag on their account or same with age verified, either vai a group setting (such as image below inspect editted) or just by creating a new join variant available to everyone.

Short:

Allow join groups Only if user has 18+ verified tag or age verified tag, Not both, Seperate options!