Windows, and any other OS will be illegal in California unless it implements this.

Right, as I said - I just don't see how this is meaningfully enforceable. It's a complete farce. It's on the level of the Online Safety Act it being enforceable.

Apple, for one, is headquartered in California.

Oh, I forgot Apple. Sure.

But there are many other OS. How on earth can they credibly enforce this?

Did you not read my comment? Anyone writing software for an OS that implements this, can be sued (in California) if it ignores the API signals from the OS and allows access to age-restricted content.

Yeah, this is just not meaningfully enforceable. Big companies will follow, but it would mostly be ignored by everyone else.