What percentile BMI standard for teens is underweight?

A BMI below the 5th percentile is underweight, and a BMI below the 3rd percentile is severely underweight. Here are all the weight ranges for ages 2-20:

100th Severely Obese

95th - 99th Obese

85th - 94th Overweight

5th - 84th Healthy

3rd - 4th Underweight

0th - 2nd Severely Underweight