Relative age in the school year and risk of mental health problems in childhood, adolescence and young adulthood

PURPOSE: Relative age within the school year (‘relative age’) is associated with increased rates of symptoms and diagnoses of mental health disorders, including ADHD. We aimed to investigate how relative age influences mental health and behaviour before, during and after school (age range: 4–25 years). METHOD: We used a regression discontinuity design to examine the effect of relative age on risk of mental health problems using data from a large UK population‐based cohort (Avon Longitudinal Study of Parents and Children (ALSPAC); N = 14,643). We compared risk of mental health problems between ages 4 and 25 years using the parent‐rated Strengths and Difficulties Questionnaire (SDQ), and depression using self‐rated and parent‐rated Short Mood and Feelings Questionnaire (SMFQ) by relative age. RESULTS: The youngest children in the school year have greater parent‐rated risk of mental health problems, measured using parent‐rated SDQ total difficulties scores. We found no evidence of differences before school entry [estimated standardised mean difference (SMD) between those born on 31 August and 1 September: .02 (−.05, .08)]. We found that estimates of effect size for a 1‐year difference in relative age were greatest at 11 years [SMD: .22 (.15, .29)], but attenuated to the null at 25 years [SMD: −.02 (−.11, .07)]. We did not find consistent evidence of differences in self‐rated and parent‐rated depression by relative age. CONCLUSIONS: Younger relative age is associated with poorer parent‐rated general mental health, but not symptoms of depression.