This is not really an aswer to your question, but since BMI charts have been brought into the discussion and this is too long for a comemnt, I'll post it as an answer. I won't get offended if someone else decides it needs to be deleted as off topic.
All of these charts start with underwater weighing as their foundation. The further they are from underwater weighing, the less useful they are. However, even underwater weighing has serious flaws - for starters it presumes that everything that is not fat has the same density. It presumes that everyone can evacuate their lungs to the same extent, and will do so uniformly on every measurement.
For underwater weighing, you are fitted with a diving belt, then you must exhale fully and hold still under the water long enough for the scale to balance. As an adult performing this exercise, I had difficulty staying still long enough to get a good reading while feeling like I was suffocating.
Charts for younger ages are extrapolated because you can't get a toddler to exhale fully and hold still under water long enough to get an accurate measure.
Most of these charts are calibrated on male athletes because that's who is attracted to Physical Education programs that do this kind of research. Nevertheless, body builders have incredibly high BMI's because they have lots of muscle, but they have very little fat. See this: http://www.militarytimes.com/articles/military-obesity-policy-bmi-tape-test
, and this: http://www.highbeam.com/doc/1P3-1308410701.html
If the BMI index can't fairly represent a cross section of the population that it is primarily calibrated to, then it's of little value anywhere else.
In my opinion, BMI charts, particularly for preschoolers, belong in the same bucket with horoscopes and phrenology.