According to study data published in JAMA Pediatrics, school-age children who were younger than their peers in the same grade were more likely to be diagnosed with attention-deficit/hyperactivity disorder (ADHD), intellectual disability, and depression.
Investigators extracted data from the Clinical Practice Research Datalink (CRPD), an electronic primary care records database in the United Kingdom. The study population included all children younger than 16 years who were registered at a CPRD-affiliated general practice. The primary exposure was relative age within the school year, calculated by comparing birth month with the grade cutoff dates in England, Wales, Scotland, and Ireland. Primary outcome measures were diagnosis of intellectual disability, ADHD, and depression. Cox proportional hazard models were used to calculate the hazard ratios (HRs) for each outcome by age group. Analyses were adjusted for socioeconomic status, calendar year, and sex.
The final study cohort comprised 1,042,106 children (51.1% boys). Median age at study entry was 4.0 years (interquartile range, 4.0-5.0 years). Compared with being born in the first quarter of the school year, being born in the last quarter of the school year was associated with diagnosis of intellectual disability (adjusted HR [aHR], 1.30; 95% CI, 1.18-1.42), ADHD (aHR, 1.36; 95% CI, 1.28-1.45), and depression (aHR, 1.31; 95% CI, 1.08-1.59).
Although the youngest age group experienced the greatest HRs for each outcome, intermediate age groups also experienced increased risk for each diagnosis compared with the oldest group. The aHRs for intellectual disability, ADHD, and depression were 1.06 (95% CI, 0.96-1.17), 1.15 (95% CI, 1.08-1.23), and 1.05 (0.85-1.29) for children born in the second quarter; for children born in the third quarter, the aHRs were 1.20 (95% CI, 1.09-1.32), 1.31 (95% CI, 1.23-1.40), and 1.13 (95% CI, 0.92-1.38), respectively.
In secondary models, no interaction was observed between relative age and sex for any outcome. However, an interaction was found between relative age and ethnicity for intellectual disability (P <.01), with children of mixed ethnicity experiencing higher aHRs than white children. However, the number of nonwhite cohort participants was small; replication in a more diverse study group is necessary to explore this association.
Relative youth in the school year was associated with increased risk for diagnosis of intellectual disability, ADHD, and depression in childhood. Clinical and policy-based approaches alike are necessary to reduce this trend.
The authors wrote, “This elevated rate may be the result of relative immaturity in comparison with peers, overdiagnosis in relatively young children, or underdiagnosis in relatively old children with intellectual disability and/or ADHD. There have been concerns about overdiagnosis of ADHD given economic costs of treatment and uncertain long-term safety of ADHD medication.” Further research is warranted to investigate whether these trends persist into adolescence and young adulthood.
Disclosure: Several study authors declared affiliations with the pharmaceutical industry. Please see the original reference for a full list of authors’ disclosures.
Root A, Brown JP, Forbes HJ, et al. Association of relative age in the school year with diagnosis of intellectual disability, attention-deficit/hyperactivity disorder, and depression [published online September 23, 2019]. JAMA Pediatr. doi:10.1001/jamapediatrics.2019.3194