Just put the age verification in the browser already.
Then introduce some new headers the browser sends to servers with some proof that the user was verified and the browser would need a response (like CORS) for it to work.
What you describe, but at the OS level, is already the law in California. We got angry about it a few months ago when it passed, do you remember?
Wrong. Try again.
Or we could have UBI and parents could parent their kids without corporations babysitting for them.
USB anal probe can be integrated the same way.