The law is not for the individual person, but for organisations that develop the OS. So they would go to Arch linux org and start threatening them with huge fines, unless they do something here. And all the headache of how exactly to implement this would fall on arch developers. At which point I suspect they would go the same route as Midnight BSD and just forbid the use in California, because as you rightly said it is impossible to implement age verification in DIY distro.