Programming

The hard part is browsers. Cookies and local storage are limited by the origin URL. You need it explicitly set on the domains you intend to visit, but those domains don't know your age. The one that knows the age is the identity provider, but it can't set it for all domains. There are other techniques that you could use, like a smart card combined with a browser extension to do local based user info attestation, but those are difficult to manage at a nation scale and I suspect people will struggle with them, though there are some countries that do have national smart cards (e.g. Estonia.)