In a large study using population-based registry data, researchers found that younger age within the school year is associated with a higher likelihood of receiving a diagnosis of attention deficit hyperactivity disorder (ADHD). Using a nationwide population-based registers, they identified all children born between January 1, 1991 and December 31, 2004 who were diagnosed with ADHD after starting school (age 7 and older). They used incidence ratios to determine the relationship between relative age within the school year and age at diagnosis. They also assessed whether there was any change over time in ADHD diagnoses (those diagnosed in 1998–2003 vs. 2004–2011).

6,136 children were identified, with a mean age at diagnosis of 9.4 years. Children whose relative age within the school year was younger were more likely to be diagnosed with ADHD; this was seen for both boys and girls. The researchers also identified a temporal pattern, with a stronger association between younger relative age and ADHD diagnosis during the years 2004–2011 than during the years 1998–2003. The association between relative age and ADHD diagnosis was independent of comorbid disorders.

Studies from other countries have also an association between relative age within the school year and ADHD diagnosis, although there are some mixed results. Specifically, epidemiological studies from countries in which prescribing rates for ADHD are higher (e.g., the United States, Canada) have shown an effect of relative age, whereas studies from countries with low prescribing rates for ADHD (e.g., Taiwan, Denmark) have been mixed. Finland has a low rate of ADHD treatment; thus, this study adds to the concern that the relative age effect is not restricted to high-treatment countries and that children who are developmentally young compared to their classmates may be more likely to have their behavior interpreted as pathological. In most countries, children within a single class can vary in age by as much as 12 months. Thus, when parents and teachers are asked to provide information to a healthcare provider, they should consider not just the expectations based on the child's school year but also the child's relative age within that school year.

Sayal K et al. Relative age within the school year and diagnosis of attention-deficit hyperactivity disorder: a nationwide population-based study. Lancet Psychiatry 2017;Epub ahead of print.