How would you impose it on them? Facebook's only way to tell your age is for you to upload an ID document and don't we want to avoid that? But when a parent buys a device for their child, they can just enable the setting that says "this device is for a child" and then Facebook can see that setting, with no further identity information transmitted. That's better privacy, not worse.

This might be plausible to an extend (probably for much younger children). If the child can manage to install an OS (which is not that difficult nowadays), or is some kind of power user, then it will not work well. Also the laws are about offline software, how it will be implemented for websites (most of the harmful social media is actually there)? There are no answers (i guess the web must implement some standard, such as http specific header). There are so many edge cases that I don't even want to talk about.