My daughter is 16 and is doing 30 minutes of cardio every day (6 days per week) on a treadmill and has changed the look of her body over the past year. She has trimmed off
excess fat and added some strong muscles to her legs. Her hard work is paying off.
Make sure to start out slowly, maybe three runs per week. After a few weeks, add another run. Then a few weeks after that, add another. This gives your body time to adapt and get stronger. One thing you would like to avoid is doing too much too fast. That is just begging for an injury. Good luck!