According to this answer from 3 years ago, it was uncertain whether porn addiction existed:
As with many behavioral addictions, including sex addiction, that would be a broader category, pornography addiction is just now starting to be researched and understood.
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In a study, Reid et al (2012) tested the proposed criteria for for the (then) upcoming DSM-5. Hypersexual Disorder was a category which would include excessive pornography use among its possible presentations. They reported:
The HD criteria proposed by the DSM-5 Work Group on Sexual and Gender Identity Disorders appear to demonstrate high reliability and validity when applied to patients in a clinical setting among a group of raters with modest training on assessing HD.
Despite that, The American Psychiatric Association opted to not include Hypersexual Behavior on DSM-V because it still lacked evidence to be considered a real diagnosis. Therefore, it is not an official diagnosis as of now, but there is sure to be more research on this and it might change in the future.
The documentary Over 18 says on its "about" page that its directors:
highlight the story of Joseph, a 13 year-old boy who is recovering from a porn addiction that he fell into when he was nine years old.
But does porn addiction exist, and if it does, could a prepubescent be diagnosed with it?