Hrm. I'm not a big fan of this proposal, but I think that implementation would be technically interesting.
Say we create a "this is a child" header, which would be sent with all HTTP requests and would mandate that the user not be tracked or advertised to. Great solution, except that everybody would immediately switch the header on because everybody hates ads. In effect, everybody wants to be a child.
So now online services fight back and say "okay, well, TikTok is only for adults anyway, you must be 18 to use it, so if this header is present, you can't use TikTok." So adults turn the header back off, and kids go back to lying about being 18.
Except for this to have teeth, we have to say that "a user's promise that they are an adult is insufficient to avoid the fine." So now we waltz into the realm of porn sites in the 1990s or Truth Social today: enter a valid credit card number to prove that you are an adult, or maybe take a photo of yourself holding your driver's license and submit it to TikTok for approval. Ick.
Okay, so let's go further. Let's say that there's a Government Identity office that, when you turn 18, grants you an Adult Citizen ID# security key that lets you verify your identity digitally. That has the upside of solving a bunch of identity problems. You could imagine using it for voting, banking, signing contracts digitally, proving eligibility to work and citizenship. But also lets you verify to websites that you're an adult. Downside: easier than ever for governments and everybody else to reliably track you and cross-reference you between databases. Less great.
But also, I'm not going to choose to "prove I'm an adult" to most of the Internet, just the sites I want to use. Proving I'm an adult is the same as telling them exactly who I am. Ain't no way I want to do that. Ads on random websites are dead. Google's dead overnight unless they require that I log in just to use it (which presumably they would, and I don't care for that at all). Now using the Internet in incognito mode becomes practically impossible.
YouTube has a lot of product placement. Is Mark Rober going to get fined if I show my kids his videos? On the other hand, Ryan and Blippi become illegal. Actually, I'm suddenly 100% on board with this.