Body mass index, a measurement calculated using height and weight, is often used to help determine if someone is overweight, underweight or at a healthy weight. Healthy BMI ranges are determined using data on the BMIs of children and adolescents at varying ages to come up with a healthy range for each given age and gender. Since the development of the BMI-for-age charts, the average BMIs have gone up significantly.
Healthy BMI Ranges for Adolescents
While there are set cutoff points for adults, adolescents' bodies are changing so much and so often that the cutoff points vary based on age and gender. Using a range is better than comparing an adolescent to the average; puberty can start anywhere from the age of 8 to the age of 14, and some children experience more rapid changes to their bodies during puberty than others.
For any given age, a measurement between the 5th and 85th percentile on the BMI-for-age charts is considered healthy. For a boy who's just turned 13, BMI is anything between 15.5 and 21.9. As adolescents get older, their BMI increases, so the healthy range for a boy who's just turned 18 would be between 18.2 and 25.7.
Girls tend to be smaller than boys, but they also can sometimes develop sooner. The healthy range for a girl who just turned 13 is between 15.3 and 22.6, and for a girl who just turned 18, it's 17.6 and 25.7.
Average BMIs for Adolescents
According to the BMI-for-age charts, the 50th percentile, the midrange for BMIs for a given age, for 13- to 18-year-old boys ranges from about 18.5 to about 22.4. For girls, the range is between 18.7 and 21.3. This isn't exactly the same as the current average BMI for adolescents, however, as the data used for these charts dates from 1963 to 1994. For example, the average BMI for an 18-year-old was 22 in the 1980s, but increased to 24.5 by the year 2000, according to a study published in the Journal of Adolescent Health in 2012.
Average BMI Changes Over Time
The number of children who are obese has been increasing over the past 40 or so years, according to the Centers for Disease Control and Prevention, but it has recently started to level off. This means that the average BMI for adolescents has been increasing as well. According to an article published in the CDC's publication Advance Data From Vital and Health Statistics in 2004, the average BMI for adolescents between the ages of 12 and 17 increased by more than 4 units between 1963 and 2002. This is because the average height only increased by 0.3 inches for girls and 0.7 inches for boys, while the average weight increased by over 12 pounds for girls and over 15 pounds for boys.
Much of this change appears to have occurred starting in the 1990s, according to the Journal of Adolescent Health study. However, beginning in 2003, the prevalence of obesity, and thus the average BMI, started to go decrease slightly, according to the Centers for Disease Control and Prevention.
Differences Between Groups
Some groups of adolescents may be more likely to have a higher or lower average BMI than others. According to the CDC, as of 2012, about 20.5 percent of 12- to 19-year-olds were obese. Asian adolescents were less likely to be obese than non-Hispanic whites, who were less likely to be obese than either non-Hispanic blacks or Hispanic youth. Adolescents with a parent who completed college were less likely to be obese than those whose parents hadn't completed high school, and adolescents who come from higher income families are less likely to be obese than those whose families have lower incomes.