Comment by vilhelm_s

The idea is that e.g. the government would give you an app that lives on your phone. When you apply for the app you provide some documents to prove your age, but you don't say anything about what sites you plan to visit. When you want to visit an age-restricted site you use the app to generate a proof that you have it, but the site doesn't learn anything more than that, and the government doesn't learn that you used the app.