No one knows why most people get cancer including bone cancer. However, despite the comments here bone cancer is often found in children and young adults . . its a type of cancer called sarcoma. There are over 60 different subtypes of sarcoma. A Sarcoma is cancer of either the bone, the muscles, the tendons, the joints, the ligaments, the fat, blood vessels, or the soft tissue in the body. It is a disease that affects the young more than adults . . so the type can be age specific.
Children, adolescents, and young adults can get several subtypes of sarcoma that attack the bone . . osteosarcoma and Ewings sarcoma are the most common types.
Adults generally have cancer that originates in an organ such as the lung or breast and than spreads to the bone . . so it is metastatic cancer and not a primary bone cancer. Children and young adults will get a primary cancer that starts in the bone.