Heh that's already what parental controls do (granted, the website don't report the content, and it's based on blacklists), but they are trivial to bypass. Even the article mention it:
> The child can install a virtual machine, create an account on the virtual machine and set the age to 18 or over
It's precisely how I worked around the parental control my parents put on my computer when I was ~12. Get Virtualbox, get a Kubuntu ISO, and voilà! The funniest is, I did not want to access adult content, but the software had thepiratebay on its blacklist, which I did want.
In the end, I proudly showed them (look ma!), and they promptly removed the control from the computer, as you can't fight a motivated kid.
a kid who can install Linux, or set up an ssh tunnel to a seedbox, is a kid who doesn't need to be told by the government what he or she should be watching
that is the job of parents/guardians
There's an ocean of difference between your device changing behavior based on a flag set by individual sites and your device using a blacklist set by some list maintainer - the main difference being that the latter is utterly useless due to being an example of badness enumeration.
> but they are trivial to bypass.
That's assuming the parental controls allow the kid to create a virtual machine. And then that the kid knows how to create a virtual machine, which is already at the level of difficulty of getting the high school senior who is already 18 to loan you their ID.
None of this stuff is ever going to be Fort Knox. Locks are for keeping honest people honest.