To be honest, a one time fee isn’t really good to prevent underage users from entering public worlds if they can already afford VRC+ for multiple months.
It would be an interesting approach but it’s not a verification system. You’ll still have the same issues just with an overall smaller amount of users since not everyone wants to pay a fee after paying for their stuffs for the club.
Only way I can see a full proof system working is with actual age verification with a photo id and worlds can mark themselves as mature audiences only (after the creator verifies themselves too). The issue with this is mainly privacy and trust in the community or VRChat the company to take care or remove your data after it is used to verify your age. Some users may not want to do this and like the open nature of other clubs.
Quite a few clubs already do this tho this is entirely another situation of stuff when a community is managing it. Online public spaces are hard to manage :(