The percentage of Latin American teenage women, including Peruvian, reporting having had sex is increasing. There are few studies in Latin America aimed at identifying the predictors of this behavior. This study uses a unique longitudinal dataset from Peru to explore which individual, family and community variables, and changes of these over time, predict sexual behavior by age 15. Results show that early sexual initiation was positively associated with being a male, having a mother with less than complete secondary education, an increase in his/her family wealth over time, being overage in school and reporting drinking alcohol frequently or sexual behavior in his/her group of friends. Peer relations were significant only for boys, while relations with parents were so only for girls. Finally, 11% of participants did not know if their last sexual partner used protection.