Annoyingly? Ironically? The best technical implementation of this law would be to make it possible for the "device owner" to tell the OS to set a flag that the user was under age. Never send the age, never send anything else. Just have a global variable indicating that the user is under age that can be accessed by the browser.
Now what would happen after that?
First oses would have to implement the above in a way that could not be bypassed, pretty much impossible if the child has access to the device.
Then you would need to require that websites honor that token or any similar token no matter how it was implemented ... https MITM etc. good luck with that.
Finally once all the implementation and enforcement hurdles are complete every website out there would immediately know that the user browsing was a child and all the trackers and ad networks on the web would immediately start targeting those users because children are marks.
Now you need even more laws and regulations to protect the children from being targeted by advertising companies, and good luck with enforcing that.