Is "effective" really the word you want to use?

Of course engineers are going to design the simplest system that meets the requirements. It seems like "privacy" wasn't on the list of requirements here.

For those in Spain I suggest protonVpn

I feel like a browser API that just gives info to the site when request of either "is under age, is of age to create an account, is adult" might be an easy way to establish something like this too

This way the site can voluntarily check if they're illegally collecting data on minors, if they're showing adult content to adults, and automatically display age appropriate content of applicable

Maybe an NSFW flag as well that sites can check to automatically show/hide NSFW content, for example on work machines or shared computers, but that's probably getting a little too finegrained

The real question is how is the age flag determined? Is it determined by the browser? The OS? Browser seems the safest bet, since Google can base it off of the Google Account, Microsoft can base it off the Microsoft account and Mozilla can shove it in the settings and potentially base it on the Mozilla account