Objective: Young relative age (ie, being among the youngest in a school class) and attention-deficit/hyperactivity disorder (ADHD) are both potential risk factors for adverse long-term outcomes. Young relative age also increases the risk of ADHD diagnosis. Using data from Swedish national registers, we investigate the independent and joint long-term effects of young relative age and ADHD on educational achievement, substance use disorder (SUD), criminality, and depression. Method: We identified a national cohort of individuals with young relative age (born November–December) and a comparison group with old relative age (born January–February). Of the total sample of 297,840 individuals, 6,528 individuals had a diagnosis of ADHD in childhood. The 4 outcomes were measured at ages 15 to 23 years. We examined main, additive, and interactive effects of young relative age and ADHD on long-term outcomes. Results: In the individuals without ADHD, young relative age was associated with increased risk of depression (odds ratio [OR] = 1.14 [95% CI =1.09–1.20]), SUD (OR = 1.14 [1.09–1.20]), and low educational achievement (OR = 1.17 [1.14–1.20]), but not criminality (OR = 1.00 [0.98–1.03]). In the individuals with ADHD, young relative age was associated with increased risk of SUD (OR = 1.23 [1.01–1.50]) and low educational achievement (OR = 1.12 [1.00–0.26]; CI included 1), but not depression or criminality (OR = 0.88 [0.73–1.07] and OR = 0.89 [0.79–1.01], respectively). An interaction emerged between young relative age and ADHD for depression (OR = 0.78 [0.64–0.95]). Conclusion: We observed relative age effects that add to the evidence supporting a more flexible approach to school starting age and that emphasize the importance of careful age-match comparisons during assessment of childhood ADHD symptoms.