I think that if we are considering technical solutions to the social problems here, the answer is well understood by professionals working in age/ID verification. You allow someone to perform a more thorough check verifying your age and identity (such as the government or your smartphone vendor), and then you use a privacy preserving protocol to provide a proof of whatever attribute (such as age) you are trying to verify in the absence of the remaining personal data. This can be as simple as exposing a "is age over {16, 18, 19, 21, 25}" api in the OS or browser, or as complex as a ZKP as needed. What doesn't work is requiring each service to independently perform verification which leads to a massive expansion in the proliferation of personal data and is incompatible with the modern approaches to data protection.

This is a solution that hopefully will cease to be relevant as soon as the remaining infrastructure is finished.