I don’t know exactly but the two big things I’ve seen, and again I’m not the engineer of it or anything, but
Your browser would have to implement some sort of 3rd party ID checker that the results could then be stored in a non-adaptable way (specifically parental controls I think would need to be set up), then when a site is loaded it reports to the browser the minimum age limit and the browser decides if you can see it or
You could register your ID on a third party ID checker site that does not log data, only verifies that you are of age. Then on load websites could then check against this third party service to verify the user is 18+.
Know that yes this is a limitation of a browser, and that’s why it’s viewed as a compromise, a word that a lot of people have forgotten. None of us really want to have to prove it, but if there is a need to prevent children from accessing content (and tbh there is a need), then I’d rather have it be done in a privacy focused way.