Attention-deficit/hyperactivity disorder (ADHD), typically affects school-aged children, but can present during the preschool years and persist into adulthood. Accurate diagnosis for preschoolers and adults requires adaptation of the current diagnostic criteria to account for differences in symptomatology across the age span. The differential diagnosis of ADHD and the pattern of psychiatric comorbidity vary with each age group and complicate diagnosis and management. To maximize outcomes clinicians must be able to accurately identify ADHD across the lifespan, and develop comprehensive, collaborative treatment plans. The Preschool ADHD Treatment Study (PATS) demonstrated the potential utility of methylphenidate for treating ADHD in preschoolers, and trials of psychostimulants and atomoxetine have shown some benefits for adults. Behavioural interventions likely have an adjunctive role in ADHD treatment for both groups. More research, however, is needed to determine the safest and most effective pharmacotherapies and psychosocial interventions for these non-typical patients.