Attention deficit hyperactivity disorder is one of the most common childhood conditions in the United States, affecting some 6.4 million children. A child with ADHD may have difficulty paying attention in school, and display bouts of hyperactivity or impulsive behavior. So far, studies have linked ADHD to genetics, a mother’s drug or alcohol use during pregnancy and exposure to toxins in the environment.

But new research published in the Journal of Pediatrics shows the age at which a child enters school may also be related to an ADHD diagnosis. A team of researchers from the National Yang-Ming University in Taiwan analyzed data involving more than 350,000 children between the ages of 4 and 17. They compared ADHD diagnoses and medication use among the youngest and oldest children in a particular school grade, taking into consideration Taiwan’s cut-off date for school entry.

The results showed that among the preschool and elementary-school students, the youngest children in a grade were more likely to be diagnosed with ADHD than the oldest children. But once the children reached middle school and high school, there was no significant difference in ADHD diagnosis among different ages. The authors say this could be because as age and maturity levels increase in teenage years, the influence of birth month may have less of an impact on ADHD diagnosis.

Because the study also raises questions about the impact of age on ADHD diagnosis, researchers say further study is needed. In particular, they want to find out more about whether the younger children may be diagnosed with ADHD simply because they are displaying immature behaviors relative to their older classmates.