According to the National Institute of Mental Health, ADHD occurs in an estimated 3 to 5 percent of preschool and school-age children. Therefore, in a class of 25 to 30 children, it is likely that at least one student will have this condition. ADHD begins in childhood, but it often lasts into adulthood. Several studies done in recent years estimate that 30 to 65 percent of children with ADHD continue to have symptoms into adolescence and adulthood.