For adults, age does not affect BMI.

More Info: For adults over 20 years of age, Body Mass Index (BMI) is calculated using standard weight status categories that are the same for both men and women and includes all ages.  BMI is a fairly reliable method to measure how much of the body if fat by using a calculation based on a person's height and weight.  Age does not play a role.

How Is BMI Calculated Differently for Children and Teens?

Though the method of BMI measurement is the same for adults, teens, and children, the interpretation of results is not.  In contrast to the standard weight status categories utilized for both adult men and women of all ages, BMI results for teens and children are based both on sex and age.  A standard unit of measurement would not be accurate because body fat differs between boys and girls and changes with age.

Is the Body Mass Index Accurate?

Many believe that the measurement of body fat using BMI can be misleading, as it does not take into consideration fat versus muscle.  This would mean that a muscular athlete with no body fat could have a higher BMI that a person with more body fat.  In the case of weight lifters, they may even have results that are considered overweight by the index.  The fact is, that for most people the BMI is an accurate measurement of body fat as research has shown that the results correlate with other more complicated body fat measurement tests such as the hydrostatic body fat test and dual energy x-ray absorptiometry.