More than 9 percent of U.S. children ages 5 to 17 were treated for mental health disorders in 2009-2011. (Source: Agency for Healthcare Research and Quality, Medical Expenditure Panel Survey Statistical Brief #440: Expenditures for Treatment of Mental Health Disorders among Children, Ages 5-17, 2009-2011: Estimates for the U.S. Civilian Noninstitutionalized Population.)
http://meps.ahrq.gov/data_files/publications/st440/stat440.shtml