Association between relative age at school and persistence of ADHD in prospective studies: an individual participant data meta-analysis

Abstract

Background: The youngest children in a school class are more likely to be diagnosed with ADHD, but this relative age effect is less frequent in older than in younger school-grade children. However, no study has explored the association between relative age and the persistence of ADHD diagnosis at older ages. The aim of this meta-analysis was to quantify the association between the relative age and persistence of ADHD at later ages. Methods: We gathered individual-participant data (IPD) from prospective cohorts that included children identified with ADHD before the age of 10 years. ADHD was defined by either a clinical diagnosis or symptoms exceeding clinical cut-offs. Our outcome was ADHD status at a diagnostic reassessment, conducted at least 4 years after the initial assessment and after the age of 10 years. No information on sex/gender or ethnicity was collected. We did a two-stage random-effects IPD meta-analysis to assess the association of relative age with the persistence of ADHD at follow-up. Findings: We gathered IPD from 57 prospective studies, conducted in 19 countries. These studies followed, for a period ranging from 4 to 33 years, 6504 children with ADHD. We found that younger relative age was not statistically significantly associated with ADHD persistence at follow-up (OR = 1.02, 95% CI = [0.99, 1.06], p = 0.19). Additional analyses revealed similar results in cohorts with a robust relative age effect at baseline. Sensitivity analyses, including those restricted to cohorts involving children with a clinical diagnosis of ADHD or with a follow-up duration of over 10 years, confirmed the robustness of our findings. Interpretation: Contrary to our hypothesis, the present study demonstrates that younger relative age is not significantly associated with decreased ADHD persistence at later ages. Alternative explanations for this result, limitations of the study, and implications of the findings are discussed.