My Abnormal Psychology professor taught his kid about sex when she was 3 because she was curious. Turns out, she and a group of friends were talking about it in PRESCHOOL. Kids are naturally curious and find out about sex much earlier that you may think. Kids also have a tendency to masturbate without realizing it, because they quickly find out what sensations and stimuli feel good and respond accordingly. It's completely natural, and whether or not they see porn is not going to affect that, because more likely than not, they will exploit some parental slip of the tongue and start asking questions (or finding out otherwise) on their own.