I used to be really skinny, buy my bmi has always been normal because I'm short. I remember at age 10, I weighed 40 lbs (18 kg) and I was 3'8" tall. At age 12 years 10 months, there was not much difference (4'4" and 55 lbs.) But at that time my parents put me on risperdal, and I weighed 100 lbs by December and was 4'7". I have lost weight since then, and know I am happily 83 lbs (37 kg) and 4' 11" 3/4. My BMI is about 16, which is not underweight, because I am 14 years and 6 months old.