This has been widely discussed, and initial implementations exist: the EU digital wallets are doing exactly this. https://ec.europa.eu/digital-building-blocks/s

This has been widely discussed, and initial implementations exist: the EU digital wallets are doing exactly this. https://ec.europa.eu/digital-building-blocks/sites/spaces/EUDIGITALIDENTITYWALLET/pages/930450954/The+Age+Verification+Manual https://ec.europa.eu/digital-building-blocks/sites/spaces/EU.... 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.