If you want the logical approach; the premise you're suggesting you want people to argue from is faulty.
The argument kicks in at a slightly deep level. Age verification is unrelated to allowing children to watch porn. Someone needs to make an argument showing that age verification would lead to blocking porn, and that argument seems to be impossible to make (age verification is a much lighter standard than the bans on torrent websites, for example, and torrenting is still as easy as it ever was). To block kids viewing porn there'd need to be a - probably global - system of censorship that exerts total control over what people can POST and GET on the internet. The likes of which we have not yet seen and would likely have catastrophically negative political consequences.
