The percentage of teenage Peruvian women reporting having had sex is increasing. There are few studies in Latin America aimed at identifying the predictors of this behaviour. 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 behaviour by age 15. Results show that early sexual initiation was positively associated with male gender, having a mother with less than complete secondary education, an increase in his/her family wealth over time, reporting having good peer relationships by age 12, being overage in school and reporting alcohol consumption and sexual behaviour of his/her group of friends. We found that peer relations were significant only for boys, while relations with parents were so only for girls. Finally, 11% of participants didn’t know if their last sexual partner used protection.