Depending on the age of the teenager a correct and healthy BMI range should be between 18.5 and 24.9.
If the teen or persons BMI is less than 18.5 then they are considered to be underweight and if the teens or persons BMI is more than 24.9 then they are considered to be overweight.
So your ideal BMI range should be between 18.5 to 24.9 for the healthy weight that the teen should be which means they are neither overweight or underweight.
If you are underweight then you need to gain some weight to stay healthy and if you are overweight you should try and lose some weight to keep your body and heart healthy.
Being overweight can lead to things such as diabetes and heart disease so keeping your weight at a healthy range can help you live a longer and healthy life.