Children who enter school at younger ages may have more difficulties with behavior and academic performance than their older peers, results from a population-based study suggest.

Elementary school students in the bottom third of their fourth-grade class for age were almost twice as likely to score low on math and language arts standardized tests, Helga Zoëga, PhD, of Mount Sinai School of Medicine in New York City, and colleagues reported in Pediatrics.

They were also more than 50% more likely to be prescribed stimulant medications for attention-deficit hyperactivity disorder (ADHD) by seventh grade compared with their older classmates (95% CI 28% to 80%), the researchers found.

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Age spans of at least 12 months within a classroom can occur based on current birthday cutoffs for school entry. “At age 5, this span accounts for 20% of the child’s age and presents a difference in maturity and performance between the youngest and the oldest child in class,” Zoëga and colleagues wrote.

This gap persisted through age 14, and should be “taken into account when evaluating children’s performance and behavior in school to prevent unnecessary stimulant treatment,” to prevent an ADHD diagnosis based on a child’s immaturity in relation to older peers, the researchers suggested.

Zoëga and colleagues analyzed data on standardized test results and filled psychotropic drug prescriptions from Iceland’s national databases. The study involved 11,785 children tested from age 9 to 12 years.

Among fourth graders, average standardized test scores were 10 percentiles lower for the youngest students, born between September and December vs. those in the oldest third of the cohort, who were born between January and April.

Rates of prescription stimulant medications were also higher for younger kids, ranging from 5.3% among the oldest third to 5.6% in children born from May to August to 8% for the youngest third.

The study researchers could not rule out the possibility of ADHD underdiagnosis among older children. They also acknowledged the results may not be generalizable to other populations, due to participants’ access to Iceland’s universal health-care system and other educational and cultural factors.


  1. Zoëga H et al. Pediatr. 2012;130:1012–1018.