I'm inclined to agree that OS-level age verification declaration is pointless and invasive but I'm sympathetic with distros looking to comply with the law. I can also see why governments want to pursue identity verification to enforce laws online even if this is obviously not the right way to achieve that.
I want to see Linux become the mainstream choice and accept that it will need to be compatible with laws like this to do so. Were Linux more widely used then organisations like the KDE e.V. might even be consulted by lawmakers which might help avoid this sort of legislation being passed in the first place!
I'm also hoping that this can be solved for those in California with some sort of optional service rather than something that gets too closely integrated with the distros or desktop environments.