The technology exists to produce a card that you own and hold, which uses your biometric (on the card not in any database) and cryptographic proof of age you got from the government to confirm the following: "the body before you has existed on earth for more than 21 years (so you can serve 'em a drink)."

All the requirements of the law would be satisfied, but the bar would get no data about you beyond >21. The government would not be able to know when you proved your age, and it would not be able to withdraw its confirmation of this basic fact about you.

How is the place of business going to confirm that the card you're carrying is actually yours? If the card does not offer any information beyond age, then people can simply use each others' cards to prove ages that aren't actually theirs.

The inevitable conclusion here is that the proof of age must be linked to your identity. The place of business must be able to verify in some way that the card is yours. Typically this is done by putting a picture on the card, and/or providing supplementary identifying information like height, weight, eye color, hair color, etc. Consequently, the bouncer looks at the picture, looks at you, determines you are indeed the owner, and then verifies your age.

The biometric version of this is no different. The card would have to contain something like fingerprint information, and you'd have to verify it with your actual finger on a scanner, to ensure that the age on the card is indeed your age and not someone else's.

Whether the identity is tracked via biometrics or by name/picture makes no difference. It's still ultimately a unique identifier that is yours and yours alone. And consequently, any surveillance would be able to know that you visited that bar at that particular day and time.

There is simply no way to devise a useful proof-of-age card that only provides age information. You really haven't thought through your claim.