> Age verification at the OS level makes no sense to me.

it's the only form of "age verification" which can be done in a somewhat privacy respecting way (as in at most leak the age)

the idea is to "bounce back" the "is old enough" decision to parent controls and let the parent choose (the Californian law doesn't quite do that perfectly, but goes into that direction)

and if you sell what is more or less a general purpose compute/internet access device with OS (which I do include phones into) I think it's very reasonable to either sell it to adults only (with a disclaimer it's "not for children") or include proper parent controls

> Most households aren't going to have a separate device for every family member

in current times in the west it is very very common for many devices to be for one person only. Especially phones, or at least have different (OS) accounts.

but again this comes back to "parent controls", weather that is for a child (OS) account or a way to switch from a child profile to a adult profile doesn't matter

but in the end, the point of such laws should be to give parents tools to parent. As well as handling the case of parent acting in neglect by inaction. But if a parent intentional decides to give their children a device with their profile because they think it's fine than that should be their choice and responsibility.

> Likewise, people generally won't create a separate account for every potential user.

where it was possible I have not seen it not used, weather it's on a switch, gaming console or PC. It is the most convenient way of automatically separates logins, browsing history, game safes etc.

and the law als isn't made for that shared computer in the living room (through it will apply there). It's more about the devices children might use unsupervised, e.g. their phone.

Every semi competent os has suppoert for multiple users

What are you talking about, most households give personal phones to their children, especially teenagers.

Laptops aren't rare either.