Valid worry, and I would prefer no such legislation, but I can picture a more optimistic outcome where this diffuses demands for more invasive and anticonsumer verification because it would somewhat address the problem of population scale psychological harm to children that there seems to be public consensus about. The sense of "something must be done" is currently giving repressive authoritarian tech an excuse to be implemented, and while there are strong arguments for why that tech is more dangerous and oppressive than it could possibly be worth, the arguments for how the problem can be addressed instead are much weaker. People often point to parental responsibility and the possibility of setting up parental control software, but this argument has some glaring weaknesses; the problem exists on a collective rather than individual level, exists despite the current possibility of parental action, and the argument does not point towards any real hope of improvement.

This all comes back to the reality that the way we use software is largely dictated by the design of that software. Defaults matter a lot. What I like about this solution is that it would work by adjusting defaults, not asking users to take extra initiative, and leaving ultimate control up to the person who bought the hardware. It would be possible, but difficult to get around it for children who can't easily acquire their own hardware, and so most of them just wouldn't, which means there is an actual possibility of it being part of an overall solution to the problem.

Whether it's the best, or a good solution, I do have some doubts about. Banning children from any participation in public discussion seems like a bad thing for a variety of reasons, and it's easy to see any sort of effective age verification going there immediately. The ability to check the OS for age category would mean an avenue for practically enforceable legislation about how online services must treat users by those categories, and most of that legislation can be expected to suck. And of course there's the risk you mention that the law is expanded to try to prevent the hardware owner from actually being in any sort of control. Still, the problem is real, and I don't think the invasive solutions are going to be defeated without proposing effective noninvasive solutions.