Background
Early sexual initiation increases the risk of HIV infection and other sexually transmitted diseases. This study aimed to determine age at first sexual initiation and associated factors among youths in south west Ethiopia.

Methods
Cross-sectional study was conducted in South west Ethiopia from January 15 – March 20, 2012. A sample of youths aged 15–24 years was taken from six health centers and three hospitals using systematic sampling technique. Cox proportional hazard regression model was used to assess the association between the outcome and explanatory variables.

Result
A total of 405 youths participated in the study and the mean ( ±SD ) age of sexual initiation was 17.07 years (±2.12). Age at first sexual initiation was positively associated with lack of employment [Adj. HR & (95% CI)?=?7.372 (1.455, 37.357)], lack of comprehensive knowledge on HIV [Adj. HR & (95% CI)?=?8.247 (2.121, 32.067)], alcohol use [Adj. HR & (95% CI)?=?3.815 (1.315, 11.070)] and khat use [Adj. HR & (95% CI)?=?7.241 (1.871, 28.016)].

Conclusion
Majority of the study participants experienced sexual initiation. Strategies should be designed to control the use of substances like alcohol and khat which were found to be responsible for first sexual initiation.