For Australia (who will need something like this this year per current legislation), the only sensible location is the government my.gov.au central service portal. None of the other services have an incentive or requirement to do it (Medicare, drivers license issuers, Centrelink). And given the scope of the rollout (all major social media, as nominated by the gov), it would need almost all of the banks or super funds to implement the same API for the project to not fail.

But I don't think anyone has told my.gov.au that needs to happen, so we are either going to get some proprietary solution from social media companies (tricky, since they will need to defend it in court as they are liable, but maybe discord saying 'best we can do sorry' or 'better than our competitors' will let them off). Or just switching off the services for a few days until the politicians panic about the blow back and defer the rollout until some committee can come up with a workable solution (ideally in the next election cycle).

I think the post office could suffice in most countries for this.

Or server operators could just implement RTA headers and put the liability on apps/devices to look for the header.

> It doesn't have to be your bank if you don't want,

"If I don't want"? I would get no choice at all about who it would be, because in practice the Web site (or whoever could put pressure on the Web site) would have all of the control over which issuers were or were not acceptable. Don't pretend that actual users would have any meaningful control over anything.

The sites, even as a (almost certainly captured and corrupt) consortium, wouldn't do the work to accept just any potentially trustworthy issuer. In fact they probably wouldn't even do the work to keep track of all the national governments that might issue such credentials. Nor would you get all national governments, all banks, all insurance companies, all cell phone carriers, all neighborhood busibodies, or all of any sufficiently large class of potentially "trustable" issuers to agree to become issuers. At least not without their attaching a whole bunch of unacceptable strings to the deal. What's in it for them, exactly?

Coordinating on certifying authorities is the fatal adoption problem for all systems like that. Even the X.509 CA infrastructure we have only exists because (a) it was set up when there were a lot fewer vested interests, and (b) it's very low effort, because it doesn't actually verify any facts at all about the certificate holder. The idea that you could get around that adoption problem while simultaneously preserving anything like privacy is just silly.

Furthermore, unless you use an attestation protocol that's zero-knowledge in the identity of the certifier, which OpenID is unlikely ever to specify, nor are either issuers or relying parties going to adopt this side of the heat death of the Universe, you as a user are still always giving up some information about your association with something.

Worse, even if you could in fact get such a system adopted, it would be a bad thing. Even if it worked. Even if it were totally zero-knowledge. Infrastructure built for "of adult age" verification will get applied to services that actively should not have such verification. Even more certainly, it will extended and used to discriminate on plenty of other characteristics. That discrimination will be imposed on services by governments and other pressuring entities, regardless of their own views about who they want to exclude.

And some of it will be discrimination you will think is wrong.

It's not a good idea to go around building infrastructure like that even if you can get it adopted and even if it's done "right". Which again no non-zero-knowledge system can claim to be anyway.

Counterproposal: "those types of social media platforms" get zero information about me other than the username I use to log in, which may or may not resemble the username I use anywhere else. Same for every other user. The false "need" to do age verification gets thrown on the trash heap where it belongs.