This is objectively not true. Not to say that a porn ban combined with age restrictions would help, but it's just objectively not true.
* Rise of incels as a thing, and even violence committed by them
* Various loneliness epidemics
* Rise of movements such as the 4B in South Korea, where women flat out refuse traditional relationships with men
* the rises in STDs and teen pregnancies can probably be explained by other factors
* The rises in various diagnosis (ADHD, etc) and rates of sexual assault can probably be explained by just having more rigorous reporting and testing, as well as higher awareness, but the rise of specific types of sexual abuse (like a popular one, choking without consent, which can easily lead to brain damage) can be directly linked to its prevalence in porn
* significant differences in opinion on equality and general political leanings between boys and girls
That's not to say that porn is a problem, and removing it for <18 will magically make everything fine. But things are decidedly messed up for a lot of teens and young adults, and parts of that messed upness can be potentially inspired by porn, and "the manosphere". The second one is more important IMO.