The requirement to check age is on the provider.
Well, if you're going that route, there are a whole bunch of other issues (apart from absolving parents from responsibility for their kids digital wellbeing). Either the provider just asks the user for their age (which is what we're doing now, and it's just 100% useless), or there would have to be some way to prove one's age. Presumably it would be via a government-issued token of some kind, ideally as a ZK proof. While there are some european countries where this is somewhat feasible, for most of the world issuing every resident a smartcard which can attest if one is an adult or not is just not possible. So, you'd be forced to either give out gov-signed certificates of age as files (which will be easily reused by kids to access stuff which they shouldn't), or you have a centralized server which can issue time-limited certificates on the fly based on some ID (which will tell the government that you wanna watch adult stuff), or you just have to upload your ID to the provider directly (I hope I don't need to explain why this is bad).
Meanwhile, what we are discussing right now is just a basic extension to parental controls. Notice how the field is not mandatory, you can just leave it empty, and I'd argue everyone who doesn't have kids should just do that. As it is implemented right now, the machine administrator decides if they want to use this or not and what to set the date to. Even if some distro complies with the stupid law and make it mandatory in california (which I'd argue they shouldn't), you can just enter 1970-01-01 and be done with it, because you decide what to put in that field during account creation.
This means any website can ask for your birth date. Then they store it, and use it for tracking. Private browser? HAH! Not your birth date!
Well, first of all, for now browsers don't even support reading userdb at all, and there's no way for website to request it. Then, I hope when it is implemented it will be hidden behind a website permission so that kids can decide if they want to share it or not (i.e. "this website wants to know you age: allow/deny"). For adults (if the california law gets implemented), I hope that "privacy" browsers will have a feature to return a random date (more than 18 years ago) every time if the field is not set in userdb, or you can just write a cronjob/systemd-timer that changes the date randomly every hour.
There is a question of random apps now having unfettered access to your child's birthday. This is indeed an issue, and poettering's approach of "just containerize it" is not very cool. It would be nice to have a way to gate userdb access behind a user prompt, similar to what I'm describing for browsers. I guess for now flatpaking everything you don't 100% trust not to read userdb is the only option.
None, sadly.