Yes and no. If you are underweight and not healthy, then that's a problem. If you are healthy and have no issues according to your doctor, being underweight shouldn't be a problem. I am underweight by twenty pounds, and I have no issues besides ability to fight colds, but hey, don't we all?
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