My opinion: The other way around puts the onus for violating the law on the OSes.

Also the telemetry is the same either way:

"Are you over 18?" Yes/No is basically the same signal to the server as "This content is rated 18+" followed by downloading more content or not.

It's also vulnerable to basically the same brute force age guessing which should be outlawed, e.g., "This pixel is for 5+, that one is 10+, ....." Until you get the user's age, but that also should be considered a violation by the website and not the OS, in my book.

Policing the OS is like policing credit card companies for kids buying cigarettes. It could work but it's not really their responsibility to check.

Also, my personal suggested approach leaves room for the OS not to disclose age and we're just back to the normal Y/N checkbox or calendar drop down on a website.

Finally, when a website tells the device the content is 18+, the device can't go "but wait, is it the kind of 18+ I'm okay with??" It'll still get blocked either way. End user effect will be the same.

Nothing about my proposal fixes fascism because they can always write a new law with more restrictions. Fascists gonna fascist, but without a working system, they can play the "protect the kids, give pornhub your full ID" card all day.