You can probably find more information than I did with a Google Scholar search for the effects of porn on adolescents and children - though this is a very hard thing to "prove beyond dispute" for several reasons - people react to media in very different ways, effects may only be activated by pre-existing patterns of behaviour and attitudes, there is a large influence from religiosity, among other things; and of course, it'd be unethical to expose children to porn in a lab setting. The best you're going to get is controlled suveys and questionnaires, from researchers working at a school, for example. Consider that we know cannabis and alcohol can have a detrimental effect on young people, but we're obviously not able to watch them smoke or drink it in a controlled environment. All that said (and note that the authors of the first paper are probably the foremost scholars of porn effects research in psychology):
"Additionally, there is meta-analytic evidence to indicate that viewing more pornography and viewing extreme pornography is associated with the sexual objectification of women and more aggressive attitudes (Hald et al., 2010)[0]. Some longitudinal findings link sexual aggression and use of violent pornography, for example, one study of 10-15 year olds in the US found that those who intentionally viewed violent X-rated materials were nearly six times more likely than others to report sexually aggressive behaviour (Ybarra et al. 2011)[1]."[2]
The research on effects of porn is chequered and often has methodological problems, replication problems, and unfortunately assumes one kind of mainstream porn (leaving out, say, erotica, fanfiction overwhelmingly written and read by teens, online erotic roleplaying, etc.) - that said, from my own amateur reading around the topic, there is a strong pointer to substantial negative effects which are usually (though not always) mediated, much in the same way alcohol consumption effects are.
[0] Hald, G.M., Malamuth, N.M., & Yuen, C. (2010). Pornography and attitudes supporting violence against women: Revisiting the relationship in non-experimental studies. Aggressive Behavior.36, 14-20. doi: 10.1002/ab.20328
[1] Ybarra, M.L., Mitchell, K.J., Hamburger, M., Diener-West, M. & Leaf, P.J. (2011). X-rated material and perpetration of sexually aggressive behavior among children and adolescents: Is there a link? Aggressive Behavior, 37, 1–18.
[2] Victoria et al. (2016). Identifying the routes by which children view pornography online: implications for future policy-makers seeking to limit viewing report of expert panel for DCMS. Technical Report. Department of Culture, Media and Sport.