The pattern of significance of the ERO and SNP factors for the onset of regular alcohol use and of alcohol dependence is different between the youngest and oldest age ranges within the entire sample, as is evident in table 2. These differences are primarily the result of differences between the populations of regular alcohol users in the two age ranges. The proportion of the at-risk sample (alcohol non-users prior to the beginning of the age range) who become regular users of alcohol increased from 15% to 43% between the two age ranges. Biological factors (genotype and endophenotype) are significant in both the onset of regular alcohol use and of alcohol dependence in the youngest age range. The prevalence of regular drinking in the oldest age range has eliminated the effect of the biological factors in its onset; only the onset of alcohol dependence is affected by biological factors. In the older age range, since it is likely that much of the onset of alcohol dependence is driven by past drinking, particularly since relatively few of those who become alcohol dependent
