Water is wet.

All for making sites to send a header with restrictions as they apply in law (age rating per location for example -- so a site could send "US:16 US-TX:18 IE:14 GB:18 DE:16" etc), and even categorise as not required in law (category=gambling or category=healthcare)

That gives the browser/app/accessing device the power to display or not display

The second part of this is to empower parents -- let them choose the age rating which can only be changed with a parental code etc. Make this the law on all consumer commercial devices -- i.e phones, macbooks, windows.

This is trivial and worthwhile.

Yes some 15 year old will build something in python in a user session to work around it as they have a general purpose computer, that's a tiny amount of the problem. Solve the 90% problem first.