[영문]Objective.To identify factors associated with age of smoking initiation and distinguish vulnerable periods for initiation according to those factors.Method.Discrete time hazards analysis was used to model smoking initiation as a function of age (5?23), demographic and familial influence variables (measured at age 13), and their interactions in a cohort of 6255 youths who completed six assessments over a 10-year period from age 13 to 23 years.Results.Half of the sample had initiated smoking by age 13, and the hazard for initiation was greatest between ages 13 and 15. In addition to differences associated with race/ethnicity, protective factors for smoking initiation included high parental education and having an intact family of origin. Having an older sibling and participant's exposure to smoking by an important adult were both risk factors, with the latter having a stronger effect for females and Blacks. For all effects, the strength of the association differed across the age range.Conclusions.The impact of demographic and family influence factors on smoking initiation varies over time. However, the maximum risk for initiation is during the early teen years, and the range of considerable vulnerability is during middle school and high school.