Adding onto what the previous poster said, another reason why teenagers are inactive and not exercising is due to a lot of hype around exercise. It's the same as people eating health food. Society, over time, has come to make it seem like you are "out of the norm" if you eat a salad over a hamburger and if you voluntarily go exercise if it isn't for some form of athletic purpose relating to an event. It's expected more that school athletes are pretty much the only people who exercise, although this is not true. Society has given a bad reputation to people trying to make themselves healthy. This has a lot to do with profit. Spreading the word around making people feel like trying to be healthy and stay active isn't fun or "cool" and instead this allows the pharmaceutical and health care industries to receive a large amount of profit. Teenagers of this generation are being raised around social media where they are being led around by all kinds of propaganda, including this.