A new study proves that the ‘terrible twos’ aren’t the only minefield in a child’s development. 

According to the Associated Press, the Journal of the American Medical Association reports that a child’s activity level takes a nosedive once they hit their teen years.  In one of the largest such studies to date, 1,000 U.S. kids of different ethnic and economic classes were monitored for six years at intervals from age 9 to 15. 

Taking place from 2000 to 2006, the study showed that while 90 percent of 9-year-olds get a few hours of exercise a day, that number plunges to an astonishing 3 percent by age 15.

The study’s lead author, Dr. Philip Nader, suspects culprits such as computers, video games, lack of recess and elimination of gym class.  Nader, a pediatrician and professor emeritus at the University of California at San Diego, worries that people won’t realize the impact of the findings: namely, that many health problems related to a sedentary lifestyle plague such children into adulthood, such as heart disease, obesity, high blood pressure and diabetes.

The findings suggest that parents should encourage their kids to get off the computer and into the yard.  The statistics showed girls to be particularly vulnerable, as their activity levels were less than boys at every age.