This is the precise question that Soatok discussed here: https://soatok.blog/2025/07/31/age-verification-doesnt-need-to-be-a-privacy-footgun/
Google recently published an open source library that proves a user’s age in a way that preserves privacy. This library is undergoing two independent security reviews, but should be production-ready in the near future.
If we’re going to force websites to implement some kind of age verification for adult content, we should demand the governments that pass these laws provide the zero-knowledge proof technologies to satisfy the law.