The line is where it empirically stops hurting the protected class. We don't do empirical evidence based policy on social stuff anymore (science in social politica is mostly rhetoric lately), so it's pointless for me to type this out, but that is the answer to establishing a boundary.
When teenagers and young adults stop measurably falling off their path in life (chosen according to their own wishes) then we can say that age is the right age for teenagers to be allowed to opt-in to porn.
This is not to condemn the average joe who is unaffected by porn, but to distance the innocent from getting addicted.
Alcohol, cars, guns and literal sex all have minimum ages. The controversy around porn is more rabid and faith-based than logical.
The government doesnt need any new tools to do it, too. Just legally require porn providers to accept a one-off credit card verification payment or 2factor phone auth or a gift card, to establish that the user is old enough to do those basic tasks. If the teenager uses a parents card or phone, the parent will find out eventually and it will solve itself.
But we all know the gov will try to implement more control than that. And that will kill the idea dead, before it begins.