No it is not wrong or a bad thing to watch porn at 13, I've known of kids as young as 7 who snuck into their parents porn stash and watched it. and they grew up to be normal undamaged young adults. however if you have been taught and indoctrinated into believing that watching porn makes you wrong, sick and perverted then yes you have a good chance of developing a guilt complex from all the negativity you've been exposed to all your life.
This is the only real damage that comes from watching porn at a young age it's nothing more then all the negative opinions of ignorant people who lack critical thinking and understanding, people who are followers more then leaders, who want to keep you under control because they choose to live under control themselves, and they can't stand to see other people indulging in the things that they choose not to.