I think there's a tradeoff triangle here, not dissimilar to Zooko's triangle or the CAP theorem, where the three aspects are age verification, privacy, and the freedom to run custom software on devices of your choosing.

You can have no system at all, which gives you freedom and privacy, but not age verification. You can have ID uploads, which give you age verification and freedom, but not privacy. You can have a ZKP-based system, which gives you age verification and privacy, but not freedom. This is because you need a way to prevent one unscrupulous ID owner from issuing millions of valid assertions for any interested user.