100 lbs is light for any 5'2"+ adult. Id dare even venture slightly lower than that but I wont for the sake of this thread.

BMI is a method used when doing just this. However, BMI should NOT be an excuse at ANY point in time to argue an eating disorder. We have established that eating disorders happen, and quite often in younger people. They find every rationale possible to excuse the fact they are the way they are. If someone is under 100 pounds and they are taller than 5'2" they SHOULD be going to a doctor. I am 5'6" and I was underweight after a surgery coming in at 136 lbs. Since then I have gained my weight back but my BMI at that time would have put me as "fine" when in reality I was not.

As long as the "the thinner the better." is the "perfect" ideal. BMI is mostly being used to justify some false bullshit that will just ignore a growing problem in both youths and adults in this society we live in.