That is already the case on most ISP's. I don't think it's legally required, but most ISP's do it with a wink wink nod bid agreement with the government.
It helps that ISP's want to do a credit check on their subscribers because then they get paid by credit checking agencies (credit checking agencies love checks for utilities because it gives a strong address to name to payment bounced-or-not linkage, so will either do the check for free, or sometimes even pay the utility for it).
So now the ISP can do a credit check on the subscriber to know their true identity, and know they are over 18, before allowing them to access the checkbox to enable porn sites.
It helps that ISP's want to do a credit check on their subscribers because then they get paid by credit checking agencies (credit checking agencies love checks for utilities because it gives a strong address to name to payment bounced-or-not linkage, so will either do the check for free, or sometimes even pay the utility for it).
So now the ISP can do a credit check on the subscriber to know their true identity, and know they are over 18, before allowing them to access the checkbox to enable porn sites.