According to the American Cancer Society, obesity is just as dangerous as smoking, when it comes to cancer risk. That's why the ACS is trying to convince young people that exercise during the teen years is a personal investment for later. Particularly, young girls who exercise are reducing their risk of getting breast cancer later in life by up to 35%.
Here's why: in women, certain types of breast cancer tumors feed off of hormones like estrogen, but exercise helps a woman's body fight this type of reaction, so the earlier a girl starts exercising, the more protection she'll get.
Boys can benefit from early exercise, too. While exercise doesn't affect their hormones in the same way, it helps keep them lean and healthy.