ADHD diagnoses increase discontinuously by a child's school starting age, with young-for-grade students having much higher ADHD diagnostic rates. Whether these higher rates reflect over-diagnosis or under-diagnosis remains unknown. To decompose this diagnostic discrepancy, we exploit differences in parent and teacher pre-diagnostic assessments within a regression discontinuity strategy based on school starting age. We show that being young-for-grade or male generates over-assessment of symptoms specifically from teacher assessment. However, under-assessments of the oldest students in a grade, especially the oldest females, account for a large part of the observed school starting age assessment gap. We argue that this difference by sex and higher school starting age effects in lower-income schools may exacerbate known gaps in educational attainment by gender and socioeconomic status. Importantly, we fail to find evidence that teachers who receive special education training make such errors.