We evaluated whether younger age in class is associated with poorer academic performance and an increased risk of being prescribed stimulants for attention-deficit/hyperactivity disorder (ADHD).


This was a nationwide population-based cohort study, linking data from national registries of prescribed drugs and standardized scholastic examinations. The study population comprised all children born in 1994–1996 who took standardized tests in Iceland at ages 9 and 12 (n = 11 785). We estimated risks of receiving low test scores (0–10th percentile) and being prescribed stimulants for ADHD. Comparisons were made according to children’s relative age in class.


Mean test scores in mathematics and language arts were lowest among the youngest children in the fourth grade, although the gap attenuated in the seventh grade. Compared with the oldest third, those in the youngest third of class had an increased relative risk of receiving a low test score at age 9 for mathematics (1.9; 95% confidence interval [CI] 1.6–2.2) and language arts (1.8; 95% CI 1.6–2.1), whereas at age 12, the relative risk was 1.6 in both subjects. Children in the youngest third of class were 50% more likely (1.5; 95% CI 1.3–1.8) than those in the oldest third to be prescribed stimulants between ages 7 and 14.


Relative age among classmates affects children’s academic performance into puberty, as well as their risk of being prescribed stimulants for ADHD. This should be taken into account when evaluating children’s performance and behavior in school to prevent unnecessary stimulant treatment.

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