Smoking among school children is becoming a serious problem in developing countries, including Bangladesh. The early initiation of smoking needs urgent intervention to protect this vulnerable group and preventing them to be addicted. This study aims to determine the age at initiation of smoking and factors affecting it. A two-stage cluster sampling was used with a selection of schools on probability proportional to enrolment size followed by stratified random sampling of government and private schools and then a random start in classes VIII, IX and X of each school targeting the students aged 13 years and above. Data collected from secondary school students using self-administered structured questionnaire. All analyses were performed with SPSS version 20.0. Missing value was treated by multiple imputations. A total of 6877 data were analysed in which 84.7% were non-smoker, 9.5% were ever smoker and 5.8% were current smokers. Among the ever smoker (n=823), 38% were current smokers, 56.5% former and 5.5% were recent quitters. The mean (SD) age at initiation of smoking was 10.9 (0.2) years. Multinomial regression analysis revealed that class grade, peer pressure and offered free cigarette appeared to be significant predictors of smoking initiation (p0.05). Promotion of smoking resistance skills among children and teens through comprehensive approaches designed to enhance general personal competence by teaching an array of personal and social life skills is recommended.