Obviously everyone here hates this, but I'm gonna offer another perspective here and prepare for the down votes I guess.
There is a very good argument for OS level age 'tracking' as a means of creating a cohesive environment for software and websites to operate without having to implement individual age verification. The biggest actual issue here is how the OS determines what the user's age is. If this is implemented similar to what California has done, the OS would simply ask for the user's age at setup, and store that value, which can then be reported to programs and websites as needed. This would allow parents to setup a device for the child and not have to separately implement parental controls on every individual conceivable program, which are often easily circumvented. This would undermine any individual website's attempts to use age verification as an excuse to collect government ID data, and the security risks inherent to that.
There's no need to put any kind of validation onto this, it should simply be self-reported.
Now admittedly I don't trust our government to implement this in any kind of reasonable way so I definitely understand and respect the outrage, but I guess I'm just trying to find some positive aspect of how this might be implemented.