Face ID verifies identity, which works if you use “alternative verification methods — a credit card on file or a government-issued ID scan — instead of the facial scan”. Basically, if you have Apple Pay through a valid credit card or Digital ID set up, your identity and age can be verified when you use Face ID. But if you don’t have either set up, you have to use age estimation, which requires cloud processing and that requires selfies because Face ID data cannot leave the Secure Enclave/Exclave.