There is actually a way to prove the age anonymously. Yubikey-like devices support attestation - they can have a private key proving authenticity of a device.
So some organization could release Yubikeys with a certain private key and distribute them in stores that allow only adult customers - like liquor stores or sex shops. Owning a key proves that one is adult without disclosing identity. Keys support USB and bluetooth and can be easily supported on any device.
Also, OS developers should implement simple parent mode - such that parents only need to flip a switch and set a password, and do not have to whitelist apps or websites - the OS should use government-provided lists. You might not like the government, but 99% of parents do not want to bother compiling white lists manually.
So some organization could release Yubikeys with a certain private key and distribute them in stores that allow only adult customers - like liquor stores or sex shops. Owning a key proves that one is adult without disclosing identity. Keys support USB and bluetooth and can be easily supported on any device.
Also, OS developers should implement simple parent mode - such that parents only need to flip a switch and set a password, and do not have to whitelist apps or websites - the OS should use government-provided lists. You might not like the government, but 99% of parents do not want to bother compiling white lists manually.