Growing up (I'm older) you had to be 18 I think to buy a playboy. I think even video based porn required you be 21 to rent/buy.
Having seen porn, I don't really think 13 year olds need to see anal gang bangs quite yet. The internet really has changed things and while I am not aware of the changes in mental health of children I'd imagine there are plenty of studies on it.
Just a random article I picked off Google. I think there is alot of things here that should be obvious and there is evidence to back it:
Pornography proves to be especially detrimental to children and adolescents. According to many researchers, early exposure to pornography is connected to negative developmental outcomes, including a greater acceptance of sexual harassment, sexual activity at an early age,acceptance of negative attitudes to women, unrealistic expectations, skewed attitudes of gender roles, greater levels of body dissatisfaction, rape myths (responsibility for sexual assault to a female victim), and sexual aggression. Children’s brains are not equipped to process the adult experiences depicted. Early exposure to pornography also increases the likelihood that depression and relationship problems develop.