Rationale: Neurodevelopment may be shaped by environmental factors such as alcohol intake. More than 20% of U.S. high school students began drinking before age 14. Adults who initiated drinking before age 14 are four times more likely to develop psychosocial and psychiatric difficulties than those who began drinking after turning 20. Little is known, however, about how the age of alcohol use onset influences brain development. Design: This study prospectively examined the effects of alcohol use onset age on neurocognitive functioning in healthy adolescent drinkers. Youth underwent a neuropsychological battery and neuroimaging session at baseline (M = 13.6 years-old, SD = 0.8), before substance use initiation, and at follow-up (M = 20.2 years-old, SD = 1.5), to evaluate changes in frontoparietal context-dependent functional connectivity (cdFC) during a visual working memory task, and neuropsychological performance. Hierarchical linear regressions examined if earlier ages of onset for first and regular alcohol use adversely influenced neurocognition and functional connectivity, above and beyond baseline neurocognition, substance use severity, and familial and social environment factors. Results: As hypothesized, an earlier age of first drinking onset predicted poorer performances in psychomotor speed and visual attention (ps<.05; N = 215) and an earlier age of weekly drinking onset predicted poorer performances in cognitive inhibition and working memory, controlling for baseline neuropsychological performance, drinking duration, and past-year marijuana use (ps<.05; N = 127). Age of first and regular drinking onset did not significantly predict follow-up frontoparietal cdFC. Exploratory whole brain analyses suggested that, as hypothesized, earlier ages of regular drinking onset were associated with higher cdFC between subcortical and fronto-temporal areas, and linked to poorer neuropsychological performance. Conclusion: This was the first study to assess the association between age of onset on neurocognition and cdFC in adolescents. Initiation of any or weekly alcohol use at younger ages are risk factors for poorer neuropsychological functioning and subcortical-cortical hyper-connectivity. This may have important implications for public policies related to the legal drinking age and prevention programming, as this study suggests that early onset of drinking increases risk alcohol-related neurocognitive vulnerabilities. Further studies are needed to replicate these preliminary findings.