Why can't I renew my U.S.?
Because YOU are applying for a passport for the first time. Prior to your sixteenth birthday, it was your parents who applied for a passport for you. It was their ID information and signatures that went on the application form, not yours. Thus you have to apply for your first passport in person. Thereafter, so long as you don't lose it or it doesn't get stolen or mutilated, you can renew it by mail every nine to ten years.