There are many possibilities for back pain even if you are young. If you have been exercising that could be one of the reasons that your back has been hurting you. You could of strained your back doing sit ups or something else.

Since I don't know you personally bit if you body is changing that could be causing you back to hurt. May be you are growing taller and the body is not yet ready for the transformation. Or if you are a girl and you are carrying a little too much in the front you back can't support the extra weight and it will bother you.

Last could be the stress factor. I heard once that now adays teens, kids and young adults have more stress lately then they have in other generations. I know that when I was either a freshman or sophomore in High School I had lower back pains because of how much I was stressing out.

In the end you may need to talk to a doctor or at least you parents. Doctors and/ or a massage therapist can probably help you get back to normal.