There are plenty of sports that teenagers can do to keep fit, football, tennis, running, walking, swimming. My granddaughter's favourite is swimming because she can go with her friends and have fun at the same time as getting exercise. It seems very fashionable to attend the gym these days where you will be advised on what exercise to take part in according to your age and ability so that you don't over do it. Many people choose this way to keep fit and it can be done from a young age.