ADHD affects 3 to 5% of preschool and school-aged children, as per the National Institute of Mental Health. As a result, in a class of 25 to 30 kids, at least one student is likely to have this disorder. ADHD usually starts in childhood, but it can linger well into adulthood. According to several recent research, 30 to 65% with ADHD continue to experience symptoms into adolescent years and early adulthood.