If a Linux distro is based on Californa, I can understand but Linux distros are generally free and under no obligation to follow California's law

I think this is going to end up in the Supreme Court as a violation of the First Amendment because it's compelled speech. Since code is speech.

Edit: Also, since Linux is open source, it can very easily be forked out.

Imagine all your deployment pipelines will fail because docker images would be considered "os" and will ask for age on each build. That also begs a question what age is a robot account?

Cant they just say "we don't support California, you should not use this if youre in California" and do nothing else?

If someone in California uses it just say "we don't support you, but here is our forums".