Banning kids from using social media would probably have a much more significant positive impact on their mental development. Obviously kids browsing Pornhub is not a good thing, but sites like TikTok expose them to much more traumatizing violent material, and of course turns them into quick dopamine-seeking zombies that are glued to their smartphones.
Frankly speaking, even for underage teenagers the most harmful thing about porn is the potential for addiction, not the content itself.