> If it becomes commonplace, the iPhone could vouch for you about your age, without disclosing your identity.
No, it really couldn't. If it can't disclose your identity, then there's no mechanism to revoke a compromised or cloned identifier, and no ability to demonstrate in court that you performed the necessary diligence correctly.
Age verification systems must, inherently, either produce a secure proof of age with no genuine anonymity, or a chocolate teapot highly insecure proof of age that's no better than a "I am over 18" tickbox that isn't going to protect websites in court from these clumsy laws.