Earlier quoted context omitted.
That's a good idea. There could be two headers, the existing RTA header that adult sites use today [1] and another static header that explicitly states there shall be no adult content. [1] - https://www.shodan.io/search?query=RTA-5042-1996-1400-1577-R... [THESE ARE ADULT SITES, NSFW]
What is adult content? I know parents who have no problem with their kids seeing porn. I know parents who give their kids a beer. I know parents who take their kids to violent movies. I used to know parents who will give their kids cigarettes. Most parents I know will disagree with their kids doing one of the above. I know songs that were played on the radio in 1960 that would not be allowed today, even though today…
I don't agree with showing actual children porn, but I also totally expect teenagers to find some way to get access to it in the age of the Internet.
Part of the challenge with this is cultural. Different places in the world think about sex, sexuality, and even the concept of what is a child differently. In the US, showing a woman's bare breasts to a person under 18 is generally considered wrong, and in many cases is illegal. In most of Europe it wouldn't even raise an eyebrow, because bare breasts are on television, sometimes in commercials even.
Set aside for a moment the question of age verification and age limits, we cannot even agree in any sort of universal sense what even qualifies as porn or adult content, and at what age someone should be able to see it. There's a difference between a 7 year old and a 17 year old seeing the same type of content, and there's also a difference between a photographic nude and a video of people engaged in coitus.
The story is basically the same for everything else you listed.
These age verification laws in many ways are trying to use the most heavy-handed mechanism possible to enforce American cultural norms on the entire planet. That's clearly wrong to do. What the GP suggested using RTA headers though puts the control into the parent's hands, which is as it should be.