Background We addressed if immaturity relative to peers reflected in birth month increases the likelihood of ADHD diagnosis and treatment. Methods We linked nationwide Patient and Prescribed Drug Registers and used prospective cohort and nested case–control designs to study 6–69 year‐old individuals in Sweden from July 2005 to December 2009 (Cohort 1). Cohort 1 included 56,263 individuals diagnosed with ADHD or ever used prescribed ADHD‐specific medication. Complementary population‐representative cohorts provided DSM‐IV ADHD symptom ratings; parent‐reported for 10,760 9‐year‐old twins born 1995–2000 from the CATSS study (Cohort 2) and self‐reported for 6,970 adult twins age 20–47 years born 1959–1970 from the STAGE study (Cohort 3). We calculated odds ratios (OR:s) for ADHD across age for individuals born in November/December compared to January/February (Cohort 1). ADHD symptoms in Cohorts 2 and 3 were studied as a function of calendar birth month. Results ADHD diagnoses and medication treatment were both significantly more common in individuals born in November/December versus January/February; peaking at ages 6 (OR: 1.8; 95% CI: 1.5–2.2) and 7 years (OR: 1.6; 95% CI: 1.3–1.8) in the Patient and Prescribed Drug Registers, respectively. We found no corresponding differences in parent‐ or self‐reported ADHD symptoms by calendar birth month. Conclusion Relative immaturity compared to class mates might contribute to ADHD diagnosis and pharmacotherapy despite absence of parallel findings in reported ADHD symptom loads by relative immaturity. Increased clinical awareness of this phenomenon may be warranted to decrease risk for imprecise diagnostics and treatment. We speculate that flexibility regarding age at school start according to individual maturity could reduce developmentally inappropriate demands on children and improve the precision of ADHD diagnostic practice and pharmacological treatment.