Governments already have everyone's ID, including DOB. They say that the problem is non-adults accessing adult sites and services. So therefore, the sites need to know that users are over 18 (or the selected government age). There should be a standardized government ID service/API that allows a person to let it disclose their age (or other user selected information) to a requesting site/service. That's all that is ne…
In theory, every EU state will have to support this soon so users can use it to verify age privately online. Still work to do to roll this out for real, but the technological part is very much already happening and I think the rollout plan is committed.