Despite a prevailing assumption that attention-deficit hyperactivity disorder (ADHD) is a childhood-onset neurodevelopmental disorder, no prospective-longitudinal study has described the childhoods of the adult ADHD population. Unexpectedly, the adult ADHD group did not show tested neurophysiological deficits in childhood or adulthood nor did they show polygenetic risk for childhood ADHD. Findings raise the possibility that adults presenting with the ADHD symptom picture may not have a childhood-onset neurodevelopmental disorder; thus, the disorder’s place in the classification system must be reconsidered, and research must investigate the etiology of adult ADHD.