It is not possible to calculate the body mass index (BMI) with just a person's age, as age does not figure in the formula for BMI. The formula for determining a person's BMI is: [weight in pounds divided by the square of the height in inches] multiplied by 703. The 703 figure is the conversion factor.
The Body Mass Index is the measure of a person's body weight in relation to the person's height. It is used to determine whether a person falls in the normal range, is underweight, overweight or obese, regardless of the age of the person. A BMI range of 18.5 to 24.9 is considered to be normal.