It'd be best to create a standard for this using wallet apps. You can obtain an age certificate from any trusted provider (decentralized chain-of-trust similar to TLS CAs), which you can then load into any wallet app of your choice on any OS, and use it with any online service which supports the standard. This should use anonymous, unlinkable(!) proofs, with the only certified data being `is_over_age`.

Though I'd prefer the way proposed by Mark Camilleri Gambin (EU politician). Have parents enable Child Mode during device setup, then expose `isMinor = true` to all websites and apps, require a parental control PIN to disable. This is a much better and cleaner solution. Requiring age verification of all adults gets it backwards.