> On an OS like Debian, does that mean a child can’t have the root password in case they use to it change the age? Does that mean we need a second password that needs to be entered in addition to the root password to change the age?

No. You're still not quite internalizing that the California regulation does not mandate any verification or enforcement or protection of the accuracy of the age bracket data. It mandates that the question be asked, and the answer taken as-is.

Which means that many of the concerns about implementation disappear, because the setting really does not need to be anything more than a simple flag that apps can check.

> Will Arduinos and similar devices also need to be age gated?

Only to the extent that they are general purpose computing devices, have an operating system, are capable of downloading apps, and are actually used by children (since the enforcement mechanism requires a child to be affected by the non-compliance). And if an app fails to obtain age information but also doesn't do anything that is legally problematic for a user that is a child, then it's hard to argue that the app's ignorance affected the child.

> Also, this doesn’t mean age _verification_ will simply go away.

It will in California, until the law gets repealed or amended. Apps won't be allowed to ask for further age-related information or second-guess the user-reported age information, except when the app has clear and convincing information that the reported age is inaccurate.