Peer influence has long been established as a robust correlate of offending, yet debate persists over whether such associations reflect a causal socialization process or merely the sorting of already delinquent individuals into like-minded friendship groups. Drawing on a nationally representative panel of Korean adolescents (the Korean Youth Panel Survey), this study examines the impact of delinquent peer associations on offending, comparing early and non-early starters of delinquency through socialization, selection, and typological (dual taxonomy) approaches. Using propensity score matching on covariates measured prior to treatment, the analysis balances adolescents with and without delinquent peers at age 15 and tracks their offending from ages 16 to 18. Results show that delinquent peers significantly impact offending across both groups, with no significant difference in effects based on the age of onset, though the influence of delinquent peers is consistently stronger among younger adolescents. Theoretical implications, limitations, and suggestions for future research are discussed.