(247) Relative age effect on ADHD diagnosis, medication prescription, and persistence: a population-based study within a birth cohort

Background: The role of relative age, referring to the difference in age among children within the same grade level, in the diagnosis and treatment of attention deficit hyperactivity disorder (ADHD) has been a topic of interest. A recent Individual Participant Data (IPD) meta-analysis, however, did not find any association between relative age and the persistence of ADHD diagnosis in late childhood, which prompted further investigation.

Objectives: To examine the impact of relative age on the diagnosis or medication prescription for ADHD in childhood and adolescence; and to investigate the relative age effect on ADHD medication use in adolescence among children with ADHD.

Methods: This is a population-based cohort study involving children born between 2001 and 2010 in Hong Kong with follow-up until 2023. Children born in November and December were considered relatively young, while those born in January and February were considered relatively old according to the Hong Kong school entry cut-off date (December 31). Cox proportional hazard regression was used to evaluate the effect of relative age on being diagnosed with ADHD or being prescribed with ADHD medication in childhood (6-12 years old) and adolescence (13-18 years old), adjusting for covariates at birth such as sex, birth year, preterm birth, small for gestational age, birth trauma, and Apgar score at 1 minute.

Results: Of 230,054 children born between 2001 and 2010, 229,343 had no ADHD records before the age of six, and 217,591 had no ADHD records before the age of 13. Compared to the relatively older group, children with relatively younger age were more likely to be diagnosed with ADHD or be prescribed ADHD medication during their childhood (hazard ratio [HR]: 1.54; 95% confidence interval [CI]: 1.44-1.65), while a smaller effect is observed in adolescence (HR: 1.13; 95% CI: 0.91-1.41). Among those having ADHD in their childhood (n=12,463), children with relatively younger ages were less likely to be prescribed ADHD medications during adolescence (HR: 0.84, 95% CI: 0.77-0.92).

Conclusions: The findings suggest that there might be a relative age effect on the diagnosis or medication prescription, and persistence of ADHD. Further research is warranted to explore the underlying mechanisms driving these observed associations and to develop targeted interventions for children with ADHD based on their relative age.