so this question is kinda vague. even if they "arent allowed to" they still can. like there are thing i shouldnt do but can still do. the real question is should it be illegal for teens. and i still would say no it shouldnt. them watching porn is not affecting your way of life so who cares? its not your business or your problem.
Who are we kidding. The teens are doing half the porn in the world. Start confiscating phones and you will see.
I consider it inappropriate for any age.