Comment by kijin

South Korea has implemented something similar, but through private corporations, not directly by the government.

When you sign up with a South Korean online service that might contain age-restricted content, you provide your name, date of birth, and phone number. The service operator uses a special telecom-provided API to have a 6-digit code sent to your phone. (The code is generated by the telecom, not the service operator.) When you enter the code, the telecom confirms the name and date of birth. No need for random online services to ask for government IDs, because they're allowed to pass the burden of proof to telecoms who have already verified it offline.

You could probably do something similar via banks, schools, the social security system, or any other regulated industry that has KYC rules.