A lot of my characters are young boys. In The Mountain City Bronzes, Kevin and his father tell their stories as children. In Beggar Charlie, Charlie tells of him and two other boys trying to find their way home. The story I'm working on presently is titled, The Last Words Of Edward Broome. In it, Albert tells his story. None of these boys are over fourteen.
Yes, I'll answer the question before the answer, it is hard to get into an adolescents minds. I am, after all, a fifty-seven year old woman. So I have to do a lot of remembering to the past when I was in school. What their interests were and such. Also, a lot of research into the past, the seventies in the first story, Victorian age in the second and the turn of the last century for my newest.
That being said, if you want a challenge, try writing from a point of view that may not be the most natural for you. It's fun and it also teaches you as a writer to be open to other's thoughts.