Ideally, it would be handled directly on the hardware. Allow people to verify their age once, using a government-run site. Then that user is now verified. Any time an age gate needs to happen, the site initiates a secure handshake with the device, and asks the device if the current user is old enough. The device responds with a simple yes/no using that secure protocol. Parents can verify their accounts/devices, while child accounts/devices are left unverified.

Government doesn’t know what you’re watching, people don’t need to spam an underfunded government site with requests every day, and age gates are able to happen entirely in the background without any additional effort on the user’s side. Adults get to watch porn without needing to verify every time, while kids automatically get a “you’re not age-verified” wall. And kids can’t MITM the age check, due to the secure handshake.