Already possible! Banks know who you are, so what if there was a safe way to let a site know that you are over 18 — and nothing more than that — through some common API?

This was exactly what the German public transport service Mopla did when I registered an account there. It needed to know my name to be able to sell me the personal Deutschlandticket. To verify my identity their web application forwarded me to list of countries, where I selected the Netherlands, and then my bank from the list there. That forwarded me to my bank's digital environment, with the request to share my name with Mopla (and just that one attribute). I then used my bank's auth system to approve sharing that claim.

Simple, transparent, and at no point did Mopla have to do anything with ID cards or AI or whatever.

I would expect systems like this to become more broadly available in the near future. In the EU for sure.