BMI under 17.5 is connected to very serious health issues, and is considered the cut off point for anorexia nervosa. Low BMIs can lead to problems with skin, hair, osteoporosis, amenhorrea (in women), reduced fertility, heart complications, renal complications, and the list goes on.
On the other hand, if you're under 21, you need to use a BMI scale adjusted for children, because the adult one will be inaccurate.