Compared with chronological age, criminologists have paid less attention to the biological sense of age typically expressed by pubertal development. Studies that have examined pubertal timing’s effects on delinquency have almost exclusively been conducted in Western countries using mostly White samples. To our knowledge, no study has ever examined this issue in the Asian context. The current study is the first attempt to bridge this research gap by examining the association among menarcheal timing, the sex composition of schools, and delinquency in a representative sample of 1,108 ninth-grade girls in South Korea. The results show that significant association between early menarche and delinquency exists only in mixed-sex schools but not in all-girls schools. In addition, the significant linkage between early menarche and delinquency in mixed-sex schools is mediated by delinquent peer associations.