This problem is always approached from the wrong angle (requiring verification of adults to view adult material) instead of the more freedom- and privacy-preserving method of requiring child-friendly sites to advertise to the browser that they are suitable for child web browsers.
What I mean by this, and the way that I would solve this problem, is to introduce an HTTP header such as X-Child-Friendly: true
or X-Content-Rating: E
and to put the onus on parents to set the child's web browser to only allow browsing sites which return this header. Every browser would need to have a "Parental Control" mode that restricts browsing to sites that return this header, but this could easily become a standard. Instead of having every adult site implement your legislative controls, now you just need child-friendly sites to add a header to their responses.
The whitelist approach is less likely to allow adult sites to slip through the net, compared to the blacklist approach.
For those who say that children would find a way around this by installing a different browser or unlocking the parental controls: it should be the responsibility of parents to monitor their child's access to the internet and installation of software. The current approach of trying to enforce age-verification on adult sites just shifts the problem to other adult sites that are not under the jurisdiction of the legislation.
Forcing age-verification for adults also has a huge bureaucratic cost and potential for abuse and loss of privacy. I think we know why legislators prefer this approach, and it isn't to protect the children.