There has been a secular decline in age at menarche since the 19th century. Early-maturing women are more likely to have their sexual debut at a younger age, which in turn gives rise to a host of reproductive health and social problems. This study used data from five waves of National Demographic and Health Surveys conducted in the Philippines between 1993 and 2013 to examine the trends and socioeconomic differentials in age at menarche and sexual debut. The changing trend in age at menarche and sexual debut was examined across birth cohorts, and logistic regressions were used to identify the determinants of early sexual debut. In the Philippines, the mean and median ages at menarche declined from 13.2 years and 12.6 years, respectively, among young women born in 1973–1977, to 12.9 years and 12.3 years, respectively, among those born in 1993–1997. The proportion who had their sexual debut by age 20 increased from 41.2% for the 1968–1972 birth cohort to 53.4% for the 1988–1992 birth cohort. Filipino women with low education, from poor families and living in rural areas were more likely to have earlier sexual debut despite attaining menarche at a later age as compared with their higher educated counterparts, and those from wealthier families and urban areas. Logistic regression analysis showed that, besides marital status, women’s education and age at menarche were important determinants of early sexual debut. However, ethnicity, place of residence and family wealth had no significant effects on age at menarche. An increasing proportion of young women were found to be having unprotected sexual debut and at a younger age, with health and social ramifications. Hence, apart from increasing the enrolment of girls in schools and discouraging teenage marriage, there is a need for social and health agencies to implement appropriate adolescent sexual and reproductive health programmes such as counselling and educational campaigns, as well as support services, to address sexual problems among the youth.