Although the EN-back and Little Man Tasks are widely used in the ABCD Study to assess cognition, performance changes across the three available timepoints remain understudied. We examined developmental trajectories of working memory, attention/vigilance, processing speed, response variability, and mental rotation/visuospatial attention. Using ABCD genetic data, we tested whether polygenic risk scores (PRSs) for ADHD relate to cognition cross-sectionally and longitudinally. The sample included 3,584 non-Hispanic White participants aged 9-14 years. Multiple linear regressions assessed cross-sectional associations between ADHD PRSs and cognition. Developmental change was evaluated using linear mixed-effects models with PRS, timepoint, sex, and parental education as fixed effects and participant and site as random intercepts. Mental rotation, attention, and working memory improved largely linearly with subtle late curvature, response variability improved more at later ages, and processing speed increased linearly. ADHD PRSs predicted cognition at specific timepoints but not developmental change, suggesting time-specific rather than sustained effects.