Copyright Goodheart-Willcox Co., Inc. Section 13-4 Balancing Calories and Energy Needs 351 For adults age 21 and older, the BMI is a number that simply relates weight to height. As shown in Figure 13-17, an adult BMI of 18.5 to 25 refers to a healthy weight. A BMI of 25 to 30 is viewed as overweight. A BMI over 30 is considered obese. For people age 2 to 20, healthy weight is interpreted differently. The reason is children and teens have different body-fatness levels as they grow and mature. Their BMI, called “BMI for Age” or “BMI for Children and Teens,” is plotted on age- and gender-specifi c growth charts. These charts are far more complex than the simple chart used for adults. Four weight classes exist for 2-to-20-year-olds: underweight, healthy weight, overweight, and obese. Body Mass Index (BMI) 13-17 Balancing calories and energy needs allows people to stay within a healthy weight range for their height.