Age of onset of daily smoking was dichotomized into early onset (onset of daily smoking at age 16 or younger) vs. late onset (onset of daily smoking at age 17 or older). Previous studies have shown that 16 vs. 17 is an appropriate age range to differentiate the impact of early from late nicotine exposure on dependence [40]. We used a dichotomous variable to represent age because the effect was hypothesized to be nonlinear in nature due to the fact that nicotine exerts distinct effects when administered during adolescence vs. adulthood [24]. We attempted to reflect this underlying causal model in our analyses by creating an age cut-score that reflected this nonlinear effect. Also, the age 16 vs. 17 dichotomy approximated a median split, which yielded near equivalent statistical power for tests within the two age samples; 46% of the sample were thus classified as early onset smokers. Research suggests that age of smoking onset can be reliably assessed with retrospective assessments [41]. To compare distinct levels of nicotine dependence, FTND scores were dichotomized into low (FTND = 0–4) and
