I actually do believe everything does harm you in at least some minuscule degree (even things that help you in a way or many, harm you in another). Even breathing does. Yet the degree of harm is not substantial enough to justify prohibition and all the downsides coming from trying to enforce it.
Being a generally normal person I also feel I wish kids see no porn yet as soon as I direct my attention to this feeling and question it I recognize it has no rational reason whatsoever, it's just as subjective as a preference can be. Banning a specific kind of content would be as reasonable as banning a food I personally don't find tasty, even if the majority feels the same - should we waste everyone's effort and sacrifice everyone's rights in such a case?
I seriously doubt seeing porn before 18 (which, by the way, is and will always be inevitable, no matter how hard we try, every interested teenager will find a way) these things сan cause any of these. And even if it could, lack of interest in real world sexuality sounds almost as great as lack of need to eat: a whole new world of possibility, autonomy, spare time and other resources instead of depending to another thing the world can use as a button to subdue you and drag you into wasteful consumption and playing unnecessary non-ergodic gambles people in sober state of mind would prefer to avoid.
What about the zombie state of mind of when you have the urge to visit a bathroom to empty your overwhelmed colon/bladder? Isn't it way more sane to just do it and forget it than to walk around obsessed with such urge for hours and days requiring others to participate in the process?
There's a "rule of a thumb" I learnt as an adolescent: as soon as you feel attracted to somebody - jerk off and cool down so you become way more sober and do less stupid things. In particular - always jerk off before going to a date: if you still feel interested - this has a chance of being a meaningful relationship, otherwise this was just a stupid hormone play. And I never really needed porn to implement this, imagination was enough.
Being a generally normal person I also feel I wish kids see no porn yet as soon as I direct my attention to this feeling and question it I recognize it has no rational reason whatsoever, it's just as subjective as a preference can be. Banning a specific kind of content would be as reasonable as banning a food I personally don't find tasty, even if the majority feels the same - should we waste everyone's effort and sacrifice everyone's rights in such a case?