There has been no research on the trends in early sexual initiation associated with socio-environmental factors in Korea. This study aimed to examine the trends of early sexual initiation with various socio-environmental factors among adolescents. The Korea Youth Risk Behavior Web-based Survey data were used, and two pooled datasets extracted from the 2006–2008 and 2014–2016 waves were conducted and compared. In this study, early sexual initiation was defined as sexual intercourse at the age of 13 years or younger. The weighted percentage and 95% confidence interval reporting early sexual initiation were estimated, and multiple logistic regression was performed for each subgroup of socio-environmental variables with the 2006–2008 pooled data as a reference. Regardless of sex, the weighted percentage of adolescents with sexual experience who reported early sexual initiation increased in 2014–2016 with statistical significance in most cases. Moreover, the possibility of experiencing early sexual intercourse overtime was more prominent among girls than boys. While indifference to adolescent sexual behavior continues, more adolescents become involved in early sexual initiation. Socio-environmental considerations, such as the establishment of safe environments for adolescents’ sexual activity, along with systematized monitoring mechanisms, should be administered.