What you describe, but at the OS level, is already the law in California. We got angry about it a few months ago when it passed, do you remember?
Note that California AB1043 isn't actually age verification; rather it just requires the OS to ask for your age, not check it.
Yes. And that's awesome. Assurance is provided by the fact that only the device owner can set the user's age. If you buy your own just select over 18. If someone else buys it for you, they can select your age or choose to fake it and you need their password to change it.
Crucially, no privacy is violated, but children are still protected. This not only protects children but also disarms any argument that we need to protect the children. It's great actually.
My sense is that many jurisdictions do not consider this sufficient, as they want to prevent parents from allowing their children to access restricted material (as happens fairly frequently). However, I guess we'll see.