At the start of Hatchet, Brian is 13 years old. His parents had recently divorced and the scars of this are fairly new on Brian.  As an emerging teen, he is having a challenging time dealing with the separation and is very much wondering why what is does not resemble what should be in his mind.  At the opening of the book, he is taking his first trip to spend time with his father in a joint custody arrangement where most of his time is spent with mother.  His emotional ordeal is compounded by the survival narrative that is to be told throughout the story.  The age of being 13 is significant to his experience as the divorce (what he will call "The Secret") and his drive to survive mark the start of his teen years.