Yes, that's even better. Make apps and websites provide an API that broadcasts the age rating of its content, then let the OS attest the apps and websites, not the other way around.

edit: on second thought, there is a trap here. If hardware manufacturers lock down the bootloader, then we're basically still handing over parental authority to governments and companies in the long run. So I think for a start, we just implement a app-install password lock like sudo. It will be easier to implement than the API. The convenience API can come later when hardware manufacturers are banned from locking bootloaders.

How would you make a website that can be over 18 or not, such as a social media feed? Would it become over 18 as soon as your following list contains a porn star (who may not have been one at the time you followed them), and then if you're under 18 you can't unfollow them because you can't load the page?