Background The age of first sexual debut among youth continues to attract interest as it marks the start of their exposure to a variety of sexual and reproductive health problems. Parental monitoring (PM) and school connectedness (SC) has been found to have an effect on this problem. Despite this fact, there is a dearth of literature on implications of PM and SC on age at first sexual debut in Ethiopia. Therefore, this paper examined the influences of PM and SC age at first sexual debut among female youth. Methods A retrospective follow-up study was conducted on 470 randomly selected female youth in Bedele town, Oromia regional state, southwest Ethiopia from February 1 to March 30, 2021. The age at first sexual debut was asked in full years for respondents who ever engaged in sexual debut at a time of data collection. Data were entered using Epi-Data version 4.6 and analyzed using Stata version 14. All variables at P-values less than 0.2 in bivariable analysis were exported to multivariable analysis. Multivariable Accelerated Failure Time (AFT) regression analyses using Weibull distribution were used to examine the association of age at first sexual debut with PM and SC at 5% level of significance. Results Overall, 132(28.1%) youths were begun sexual activities of which 79.54% was an early (vaginal penetrative sex before 18 years old). The incidence rate was 15.58/1000 PY with 95% CI of [13.14, 18.47]. The result shows that, female youth start first sexual debut as early as 10 year and mean age was 16.89±2.82. Only 27.97% of those who began sexual debut used contraception during first sex. Multivariable Weibull AFT regression analyses adjusted for different variables showed that high PM (Adjusted Time Ratio (ATR) 1.13: 95%CI; [1.04, 1.21]) and good SC (ATR: 1.14: 95% CI; [1.06, 1.22] significantly delays the age at first sexual debut. Conclusions Overall, four out of every five premarital sexual activities were early sexual debuts. High PM and high SC significantly decrease early sexual debut by delaying the age of sexual initiation. Therefore, family and school involvement focused on PM and SC of the youth is recommended as an important mechanism for preventing youths’ risky sexual behaviour, including early sexual debut.