Is someone at 5'5, 16 years old, and weighing 90-91 pounds normal, overweight, or underweight?

15.1: The text that goes with the bmi chart probably defines underweight to be 18.5 or less. The bmi was a population measure that was not intended to be applied to individuals; in real science, waist size correlates much better with cardiac risk. Junor may want to build muscle doing aerobic / resistance training but many track stars are "underweight". If he can do sports, his weight's probably ok.