Objective: To examine gender and age differences in attention-deficit/hyperactivity disorder (ADHD) symptom endorsement in a large community-based sample. Method: Families with four or more full siblings ascertained from Missouri birth records completed telephone interviews regarding lifetime DSM-IV ADHD symptoms and the Strengths and Weaknesses of ADHD-Symptoms and Normal-behavior (SWAN) questionnaire for current ADHD symptoms. Complete data were available for 9,380 subjects aged 7 through 29 years. Lifetime and current DSM-IV-like ADHD diagnoses were assigned by the DSM-IV symptom criteria. Linear regression was used to examine sex and age effects on SWAN ADHD symptom scores. Logistic regression was used to examine sex and age effects on specific ADHD diagnoses. Fractional polynomial graphs were used to examine ADHD symptom count variations across age. Results: Overall prevalence of current DSM-IV-like ADHD was 9.2% with a male:female ratio of 2.28:1. The prevalence of DSM-IV-like ADHD was highest in children. Gender differences in DSM-IV-like ADHD subtype prevalences were highest in adolescents. On average, individuals with lifetime DSM-IV-like ADHD diagnoses had elevated current ADHD symptoms even as adolescents or adults. Conclusions: Lower male:female ratios than reported in some clinic-based studies suggest that females are underdiagnosed in the community. Although they may no longer meet the full symptom criteria, young adults with a history of lifetime DSM-IV-like ADHD maintain higher levels of ADHD symptoms compared with the general population. The use of age-specific diagnostic criteria should be considered for DSM-V and ICD-11. (Contains 2 figures and 6 tables.)