Body mass index cannot be calculated using only a person's age, according to the Centers for Disease Control and Prevention. BMI is calculated using a person's height and weight.
To calculate body mass index, divide the person's weight in pounds by his height in inches squared. Then, multiply that figure by 703, as the Centers for Disease Control and Prevention explains. This formula is used to calculate the BMI for all adults regardless of age. However, with the same BMI, a woman is likely to have more body fat than men, and the elderly are likely to have more body fat than younger adults.