Earlier quoted context omitted.
That was, in fact, what COPA mandated in the US in 1998, and SCOTUS struck it down as too onerous in Ashcroft v. American Civil Liberties Union, kicking off the last 20 years of essentially completely unregulated Internet porn commercially available to children with nothing more than clicking an "I'm 18" button. At the time, filtering was seen as a better solution. Nowadays filtering is basically impossible thanks to…
This has already come up before the Supreme Court, with the argument that filtering was a less invasive technique to fulfill the government’s legitimate interests back in the early 2000s. That ship has sailed. Even the opposition admits that trying to get everyone to filter is not going to work and is functionally insignificant. The only question is whether age verification is still too onerous.
You've skipped right past the "does it work" question. It doesn't. Porn is available on file sharing networks in far greater quantity than it is on reputable websites.
The only realistic methods I'm aware of are whitelist filtering, sufficient supervision, or sufficient interaction and education.