Well the implementation is going to be that you prove your age to the id app that only saves the info that you’re over 18. Then the id app just tells that info to the website (or app or whatever I guess). So the site doesn’t know who is trying to prove their age or even what their age is, just that they’re over 18.
It would be nice to have that sort of quick and easy app for verifying your personal information too. I don’t think we have one where I live. We always use a bank for that and that’s more hassle than I’d like