I mean, great? Most mainstream devices do however, whether it's an AppleID, Google account or Microsoft account.
Quicky
I've seen this suggested elsewhere and it seems like the least intrusive suggestion to me - why not simply use the device as the age verification. Almost every phone/tablet/computer already knows your age through it's own sign-up/activation method, so why not allow the device to offer an API that provides age verification to sites that require it.
It could simply be a permissions-based answer where an adult site requests a yes/no answer to the question "is this user an adult" from the device and the user is prompted to provide the permissions for the site to have that data.
This would solve the problem for the vast majority of iphone/android/windows/macos consumers.
I spent a lot of hours in F-19 Stealth Fighter on the Atari ST as a kid. The manual for that thing was unbelievably in-depth. I'm utterly convinced I could fly real combat missions in that (fictional) plane.
Jesus Christ, no, I'm not suggesting that nothing changes from exactly what we do now. I'm suggesting a new, more secure, less intrusive method, and it's not even an original suggestion. Just try a little bit of thought.
If it's going to be implemented by law anyway, the age verification should be at the device level. The device accounts already do ask your age - directly or indirectly - although it's not stringently enforced, however each of the big 3 already have a minimum age requirement to set up an account as per their terms and conditions.
It's not a big leap to suggest that true age verification is done at that point seeing as you already often have to provide an age or payment information to set up on-device payment details, meaning there's no need to involve a third party at any other subsequent point.