EU published demo video exactly how age verification system works (as well as code, as well as documentation of everything): https://youtube.com/watch?v=ULFTrTznG7Y
They claim “The data privacy of the user is fully respected. The citizens can be sure that there is no tracability of their activities, no user profile can be generated”
They also say anyone (like member states) can deploy this solution, and they can customize to their needs depending on national law and use-cases 
Solution is based on EU Digital Identity Wallet - Wikipedia technology.