Any ID has to reveal enough info to reasonably convince the other party that the ID belongs to that person.

These threads always bring up a hypothetical digital ID that simply says “over 21”, but it’s missing the key point that the ID needs to also give enough information to reasonably tie the identity to the user. Otherwise everyone underage would run around with borrowed or stolen IDs because there was no way to prove it did or did not belong to them.

In theory a digital passport could reveal age 21 or older with a photo and name, but it’s only marginally less info for a lot more complexity.

There are solutions to this. Look at how state ID on iOS is handled.

There’s an enrolment process where your identity is bound to your phone, and secured using biometrics.

When you need to prove age, the device can produce a signed token attesting to fact that your older than 21 etc. and your device is trusted to validate your identity using a biometric scan performed by your phone.

All of this is dependent on everyone trusting your phone to both validate your physical identity before signing something, and also not sharing anything it shouldn’t. But given you can already enroll US state ID on iOS, those problems are clearly solvable.