Expectations about alcohol's effects and perceptions of peers' behaviors and beliefs related to alcohol use are each shown to strongly influence the timing of drinking onset during adolescence. The present study builds on prior work by examining the conjoint effects of within-person changes in these social-cognitive factors on age of adolescent drinking onset. We related youths' alcohol status (i.e., alcohol-naive, initiation during study, prior initiation) to increases in positive and negative alcohol outcome expectancies (AOEs), as well as increases in perceived peer/close friend alcohol use and acceptance, during adolescence. We also investigated whether changes in AOEs and perceived social norms prospectively predicted alcohol onset in alcohol-naïve adolescents. Participants were 1,023 adolescents aged 12.2 years on average at enrollment (SD = 0.98), 52% female, participating in an ongoing longitudinal survey on substance use and health behaviors. Positive AOEs, close friends' norms, and same-age peer norms increased linearly, whereas negative AOEs decreased linearly. Changes were attenuated for participants who remained alcohol-naïve and increased for participants who experienced initiation during the study. Furthermore, we found associations between individual changes in AOEs and perceived social norms. Finally, survival models revealed that onset of alcohol use was prospectively predicted by stronger initial positive AOEs, as well as increases in close friends' norms and decreases in negative AOEs over time. These findings emphasize codevelopment of AOEs and perceived social norms, coinciding with, and predictive of, onset of alcohol use, and point toward a unique role for within-individual changes in identifying youth at risk for early onset of alcohol use.