http://www.cdc.gov/nccdphp/dnpa/bmi/bmi-for-age.htm
In children and teens, body mass index is used to assess underweight, overweight, and risk for overweight. Children's body fatness changes over the years as they grow. Also, girls and boys differ in their body fatness as they mature. This is why BMI for children, also referred to as BMI-for-age, is gender and age specific.1, 2 BMI-for-age is plotted on gender specific growth charts. These charts are used for children and teens 2 – 20 years of age. For the 2000 CDC Growth Charts and Additional Information visit CDC's National Center for Health Statistics.
Each of the CDC BMI-for-age gender specific charts contains a series of curved lines indicating specific percentiles. Healthcare professionals use the following established percentile cutoff points to identify underweight and overweight in children.
Underweight BMI-for-age < 5th percentile
At risk of overweight BMI-for-age 85th percentile
to < 95th percentile
Overweight BMI-for-age > 95th percentile
BMI decreases during the preschool years, then increases into adulthood. The percentile curves show this pattern of growth. What does it mean if my child is in the 60th percentile?
The 60th percentile means that compared to children of the same gender and age, 60% have a lower BMI.
Example
Let's look at the BMI for a boy as he grows. While his BMI changes, he remains at the 95th percentile BMI-for-age.
Age BMI Percentile
2 years 19.3 95th
4 years 17.8 95th
9 years 21.0 95th
13 years 25.1 95th
We see how the boy's BMI declines during his preschool years and increases as he gets older.
Boys 2 to 20 years old 
Why is BMI-for-age a useful tool?
BMI-for-Age is used for children and teens because of their rate of growth and development. It is a useful tool because
BMI-for-age provides a reference for adolescents that can be used beyond puberty.
BMI-for-age in children and adolescents compares well to laboratory measures of body fat.
BMI-for-age can be used to track body size throughout life.
For the 2000 CDC Growth Charts and Additional Information visit CDC'S National Center for Health Statistics.
1 Hammer LD, Kraemer HC, Wilson DM, Ritter PL, Dornbusch SM. Standardized percentile curves of body-mass index for children and adolescents. American Journal of Disease of Child. 1991; 145:259–263.
2 Pietrobelli A, Faith MS, Allison DB, Gallagher D, Chiumello G, Heymsfield, SB. Body mass index as a measure of adiposity among children and adolescents: A validation study. Journal of Pediatrics. 1998; 132:204–210.
Date last reviewed: 12/16/2004
Content source: Division of Nutrition and Physical Activity, National Center for Chronic Disease Prevention and Health Promotion