They have it written in a very confusing way on the CDC site.  I had to read it a few times to follow what they were doing.  For adults, BMI is a set formula using kg/m2.  However, using the same formula won't give you usable numbers in children.  How it works is that you calculate the BMI like you would for an adult, then compare that number to the chart that shows how the BMI ranks for their specific age.  A committee got together to decide the correct healthy weights for each gender for their ages. The Obesity Action Coalition explains how childhood obesity is classified a little more clearly. But you are correct that the height and weight are compared to others of the same age.  The height one is fine and can stay were it is, but they really need to stop comparing by weight since there are more fat kids now than ever before in history.  At least the BMI-to-age charts take that into consideration.