Background Previous research on the etiology of ADHD symptoms suggests that neuropsychological differences may be present as early as birth; however, the diagnosis is typically not given until school age. This study aimed to (a) identify early behavioral and cognitive markers of later significant parent and/or teacher ratings of ADHD symptomology, (b) examine sex differences in these predictors, and (c) describe the developmental trajectories of comorbid symptoms in school‐aged children. Methods 1,106 children and at least one parent enrolled in the NICHD Study of Early Child Care and Youth Development were followed from 1 month of age through 6th grade. Effect size calculations, discriminant function analysis, and growth curve analyses were conducted to address the three aims. Results Children with high‐ versus low‐ADHD symptomology at 3rd grade could be distinguished using cognitive and behavioral measures as early as 15 months (females) and 24 months (males). Sensitivity and specificity were modest at 15, 24, and 26 months. Growth curves revealed significant differences between high‐ and low‐ADHD groups in comorbid symptoms at kindergarten and significantly different slopes for externalizing, social skills, and academic skills ratings across elementary school. There were few gender differences on cognitive and behavioral variables within the high‐ADHD group. Conclusions Cognitive and behavioral markers of ADHD symptoms are present in children prior to entry into formal schooling, but current behavioral screeners are not developmentally sensitive to these differences in infancy and toddlerhood.