Very straightforward with existing primitives. Use Login.gov to attest about age range to consuming service as Apple does today. I don’t think there is will to implement this at Login.gov at the moment though.

If you have a Social Security or CBP account for TSA Precheck or Global Entry, you have this account today. IRS will eventually adopt when stronger remote identity proofing requirements have been met, replacing id.me. Important context because it means it’ll cover everyone 18+ in the US eventually.

https://www.login.gov/what-is-login/

https://support.apple.com/en-us/125662

(Login.gov also supports state use cases as of 2024)