BACKGROUNDTo examine the association between age at first sex and the prevalence of sexually transmitted infections (STIs), including HIV-1 and HSV-2, in women in Moshi urban district, northern Tanzania.METHODSA total of 2019 women aged 20 to 44 were randomly selected in a 2-stage sampling from Moshi urban district, northern Tanzania. Information on demographics and sexual behaviors were obtained. Blood and urine samples were drawn for testing of HIV-1, HSV-2, and other STIs.RESULTSWomen who had their first sexual intercourse at age between 18 and 19 (OR = 0.66; 95% CI = 0.50-0.86) or 20+ (OR = 0.46; 95% CI = 0.36-0.60) were less likely to have STIs, including HIV-1 and HSV-2, than women who had their first intercourse before their 18th birthday. The hazards of having had first sex at an earlier age were significantly higher for women who tested positive for STIs (HR = 1.52; 95% CI: 1.37-1.69) or had STI symptoms (HR = 1.17; 95% CI: 1.05-1.30). Early age at first sex was associated with having a regular noncohabiting partner (HR = 1.40; 95% CI: 1.23-1.58), female circumcision (HR = 1.20; 95% CI: 1.02-1.40), and coercion at first intercourse (HR = 1.47; 95% CI: 1.15-1.89).CONCLUSIONSIn sub-Saharan Africa, where the prevalence of HIV and other STIs is high, a better understanding of the determinants of the age at first sex is crucial for HIV/STI prevention programs. Prevention programs should not only aim at delaying the age at first sex but also address factors leading to early age at first sex.