Calculating the BMI of an individual is straight forward:
Divide the weight in kilograms by the square of the height in meters.
The interpretation of a BMI value for adults (for this purpose the CDC defines an adult as someone age 20 or greater) is also straight forward:
BMI Weight Status Below 18.5 Underweight 18.5 – 24.9 Normal or Healthy Weight 25.0 – 29.9 Overweight 30.0 and Above Obese
Interpreting the average BMI of a group of adults is also straight forward: average the individual BMI values and use the above chart to interpret the average BMI (i.e., on average the group of adults is Underweight, Normal or Healthy Weight, Overweight, or Obese).
However, the interpretation of the BMI for a child (the CDC defines a child as someone older than 2 years of age but younger than 20 years of age) is a little more complicated. The BMI value for the child is referenced to a gender specific chart which contains BMI-for-age percentiles for different ages. The BMI-for-age percentile indicates the interpretation:
BMI Weight Status BMI < 5th pctile Underweight 5th pctile <= BMI < 85th pctile Normal or Healthy Weight 85th pctile <= BMI < 95th pctile Overweight 95th pctile <= BMI Obese
I would like to calculate an average BMI that I can then interpret as I did for adults, but I am struggling to define how to calculate and interpret the average BMI for a group of children that may include both genders and has children of varying ages. I cannot take the average BMI value and compare it to a chart because the individual BMI values correspond to different ages and genders.
EDIT (To answer Glen_b's questions and provide more context):
I am looking at a small region (think 3-4 counties worth of area in the United States) of the country divided by Zip Code Tabulation Areas (ZCTAs). For each ZCTA, I would like to measure the prevalence of overweight and obese people in that ZCTA. This will provide some evidence as to why one ZCTA is more suited to and will hopefully benefit more from a targeted intervention to reduce obesity than another ZCTA.