Age is not factored into body mass index calculations; the calculations are based only on height and weight, according to the Centers for Disease Control and Prevention. However, when interpreting the results for children and teens, the CDC uses BMI-for-age growth charts to take age and gender into account.
Age and gender are not considered when interpreting BMI results for adults, notes the CDC. The CDC BMI charts do consider the differences in body fat amounts between girls and boys, and the changes in their body fat amounts as they age. The agency's charts draw the distinctions by depicting BMI as a percentile ranking. Using the reference population from 1963 to 1994, children ages 2 to 19 years are classified as obese if their BMI is at or above the 95th percentile of children of the same age and gender.