I’ve been noodling on this idea for a while but I think getting commercial acceptance would be hard. People have tried it with crypto albeit with lukewarm results. I think to have the network effects required to be successful in such an endeavor, it would have to come from a vendor like apple or google unfortunately.

You kind of want an mTLS for the masses with a chain of trust that makes sense.

mTLS is no good because the target service could then uniquely identify you. I think you explicitly want a three-party scheme where the target service just accepts the idp's assertion about your age in a cryptographically secure way.