<p>Children who start school at a younger age are more likely to receive pharmacological treatment for attention deficit hyperactivity disorder (ADHD). This pattern is widely attributed to peer-comparison biases, but the relative importance of different underlying mechanisms remains debated. We use administrative data from England to study how school starting age affects ADHD prescription rates from ages 5 to 15. Exploiting a sharp school-entry cutoff and comparing children born just before and after September 1, we estimate both age- and grade-specific effects of early school entry. We find that early starters are 40–50% more likely to be treated for ADHD by age 15. This long-term gap is entirely driven by first-time prescriptions among children aged 5 to 8. Using a novel decomposition strategy, we isolate the roles of relative age, absolute age, the length of school exposure, and age at school entry. We find that relative age is the primary long-term driver, aligning with previous evidence on peer-comparison bias, while absolute age and the length of school exposure have only short-lived effects. Our findings suggest that early starters are more likely to receive marginal diagnoses due to their relative immaturity, and we recommend refining diagnostic guidelines and classroom grouping practices to reduce diagnostic gaps.</p>