Two independent studies have found that students whose birthdays fell just before their school's age enrollment cutoff date—making them among the youngest in their class—had a substantially higher rate of ADHD diagnoses than students who were born later. One study, using data from the Early Childhood Longitudinal Study-Kindergarten cohort, found that ADHD diagnoses among children born just prior to their state’s kindergarten eligibility cutoff date are more than 60% more prevalent than among those born just afterward (who therefore waited an extra year to begin school). Moreover, such children are more than twice as likely to be taking Ritalin in grades 5 and 8. While the child’s school starting age strongly affects teachers’ perceptions of ADHD symptoms, it only weakly affects parental perceptions (who are more likely to compare their child with others of the same age, rather than others in the same class). The other study, using data from the 1997 to 2006 National Health Interview Survey, found that 9.7% of those born just before the cutoff date were diagnosed with ADHD compared to 7.6% of those born just after.
The two findings suggest that many of these children are mistakenly being diagnosed with ADHD simply because they are less emotionally or intellectually mature than their (older) classmates.