Targeting them has a lower return than targeting people who are of age.
Specific to targeting teenagers, porn users are mostly under 30, so your statement about 14-18 being around the same as 18-80 is irrelevant. About not being able to pay at 14-17, sure, let's go with that, but they start paying after that. We are talking a few years here. Companies do have incentives to create customers just a few years down the line.
Let me ask a few rhetorical questions. How do you think social media became popular? How did tiktok, snapchat, facebook, instagram become popular? Even yikyak became popular because they targeted teens. Why has so much advertising been targeted to teenagers for decades now? Hell, how did onlyfans become popular? All these companies have massive incentives to target teenagers.
So your stance of these porn companies not targeting teenagers is just being willfully obtuse.
Porn companies don't have any kind of monopoly or brand loyalty and the ones shady enough to do something like that are exactly the ones that won't still be in business by the time today's kids are adults, so anyone doing it wouldn't be the one deriving a benefit from it.
Even normal companies don't care about customers decades from now because the thing they do teach in business school is discount rates. A dollar in 10 years is worth less than half that today. Likewise, managers get bonuses and promotions on the basis of present-day profits rather than something that happens a generation from now when they're likely to be at a different company anyway.
The premise that they're expected to do that on a widespread basis is ridiculous. Instead it will be one fool who writes something along those lines in an email which is then published because media companies love publishing anything which is bad PR for someone they don't like, regardless of whether it was ever widely implemented or implemented at all. It isn't an actual business strategy for real businesses.