Not sure if you know this but in a lot of EU countries people have identity cards that are already smart cards and are used to digitally sign documents (with full legal validity) and login into public services.

Adding an extra certificate for the age verification would be very easy. They already have all the certification infrastructure in place and are used to it. However... like I said before it looks they really want to control where you validate your identity so they won't do it in this way.

There's also another thing to consider: a US citizen would never agree to have a unique digitally signed ID issued by the local state with an intermediate certificate issued and controlled by the federal govt. Note that if they implement the same model the EU is following both the local state and the federal govt would be able to revoke those ID (certificates) at any time.

People say that the US is turning into surveillance / china-like state but in reality the EU is way, way closer than that. Just look at what was done with the EU Digital COVID Certificate (EUDCC) recently:

The EUDCC was a digitally-signed document.[1] It was usually supplied in the form of a QR code, either contained in a PDF file, or as a printout. There are various mobile apps available to store and display the EUDCC (such as the Corona-Warn-App); alternatively, the EUDCC can be presented on paper.

Technically, the QR code contains a JSON document with the information payload. This JSON document is serialized using Concise Binary Object Representation (CBOR), and digitally signed according to CBOR Object Signing and Encryption (COSE). The resulting data is compressed with zlib and encoded into the final QR code

And yes, there were countries blocking you from going into a store to buy basic stuff without showing a valid COVID certificate. No vax or no proof of recovery = starve out. Add the inability to move between cities to that and you're very, very close to the "democratic" China.

More here: https://github.com/ehn-dcc-development/eu-dcc-hcert-spec