I take your point, but it's extremely foolish to think that ensured compliance won't be sought when it's technologically feasible.
Its already illegal to show adult material to minors. If they were about enforcing laws properly they'd just do that. For instance a court ruling that adult material isnt allowed on the internet, since you can't effectively ID visitors.
Anyone could just make a fake ID though, so what did they do? Make creating fake IDs a crime. If we can lie about our age to the OS, they'll make it illegal to do that as well.
This is an obvious move to reduce computing freedom.
There is already a pre-existing push to bring attested computing to Linux.
The California law is not the only OS level age verification law passed or in the process of passing in the US.
One wonders if you're being intentionally obtuse.