Key Takeaways: In a nationwide Finnish cohort of more than 580,000 individuals, later age at ADHD diagnosis was consistently associated with lower GPA at aged 16 years. Individuals diagnosed with ADHD in adolescence were less likely to complete academic upper secondary education and more likely to experience school dropout by aged 20 years. Female patients were diagnosed later than male patients on average, and delayed diagnosis was linked to reduced academic progression across multiple educational milestones.