Age verification could indeed be implemented in other ways. The approach outlined above is for information verification and trust projection in general, meaning you can put just about any verified information on a certificate and it can be used online.

Here is a concrete example of how trustworthy certificates can be used online, this is my personal profile on bluesky with verification that is independent of the Blue sky service: https://bsky.app/profile/bitlooter.bsky.social

If you click on the profile image you can enter that code into https://certisfy.com/app to verify the identity of the profile. That sticker could be on any online profile to prove high quality authenticity, it could for instance be on an e-commerce site to prove that the site isn't a scam.