> unless they get more complicated with it by dynamically adding it to individual videos say, on Youtube

YT already does this. I never watch YT signed in, and I often see videos that require you to be logged in as the video is age restricted.

Agreed though in my example the point would be to set the header in the case the child is logged in but for whatever reason the site does not know their age. Instead of a third party site, a header is sent with the video tagged as adult that triggers parental controls if they are enabled by the device owner.