> That makes sense for purely offline media playback, but how could that work for a game or application or website? Ship several versions of the app for the different brackets and let the OS choose which to run?

Suppose you had an ID requirement instead. Are you going to make two different versions of your game or website, one for people who show ID and another for people who don't? If so, do the same thing. If not, then you have one version and it's either for adults only or it isn't.

> You'd also not be reporting your age, you'd be sending a "please treat me like an adult" or "please treat me like a child" flag.

Except that you essentially are reporting your age, because when you turn 18 the flag changes, which is a pretty strong signal that you just turned 18 and once they deduce your age they can calculate it going forward indefinitely.

This is even worse if it's an automated system because then the flag changes exactly when you turn 18, down to the day, which by itself is ~14 bits of entropy towards uniquely identifying you and in a city of a 100,000 people they only need ~17 bits in total.

The alternative wasn't an ID requirement, the alternative was the client/OS sending the flag to the server/app.