Background: Early age at drinking initiation may be one factor responsible for underage drinking in Thailand and this may be affected by cultural and generational changes over certain periods of time. This study aimed to explore the effects of time period and generation on drinking onset of young Thai drinkers in the past decade. Methods: We analyzed data from a total of 60,018 Thais aged 15 to 24 years from 4 national surveys conducted in 2007, 2011, 2014, and 2017. We used multilevel and event history analysis to examine the effects of period (survey year) and cohort (birth year of participants), adjusted for sex, age, area of residence, and region on drinking onset. Results: The overall prevalence of past-year drinking was 23.6 % (95% confidence interval [CI]: 23.2, 23.9) among participants of all surveys. Cox proportional hazards model revealed significantly different probabilities to have initiated drinking between participants of different surveys and birth years, indicating significant period and cohort effects. After adjusting for sex, age, cohort, living region, and area of residence, participants in the 2014 survey had the highest likelihood to have started drinking (hazard ratio [HR]: 1.25 CI: 1.15 to 1.36), compared to the participants of the same age in the 2007 survey while those in