As an adolescent, your body is growing and developing into an adult, becoming stronger and faster. Just about every 13 year old is able to run faster than he could when he was 12.
For each person, though, this experience is different. Some people's bodies mature earlier than others. So compared to your peers, it could be that, if you have an early growth spurt, you are faster than your friends now, where last year you were slower. Next year, they may catch up.
Exercise will not change the timing of your maturity, but it will definitely help you build muscle, speed, and endurance. If you are serious about running, you can't count on being faster every year without training.