It doesn't have to be exclusively digital. You can be psuedoanonymous using some form of key as verification. To get a key, you have to present your ID in person at, for example, the social security office or local DOL.

All the key does is attest that "this person is over X years old" with no other identifying information associated with it.

I think blending in person & digital together is going to be the best way forward. Like going to the store and buying alcohol. I have little privacy risk from the cashier glancing at my ID for a second to check my birth date.

But that would require the government to set up the system that lets you present your ID and get a key. They haven't done that, so it's not valid to blame businesses for not using it.