Information for the study was collected from a self-report study based on a sample of 7th and 11th grade students from four junior and senior high schools in a north-central county of Illinois. Administered in the spring of 1987, 456 usable surveys were completed. The independent variables included age, gender, time with family, time with friends, and church attendance. Age was measured directly as the respondents' real age. The dependent variables included measures of the frequency of use of alcohol, marijuana, and hard drugs. Each of the items used a reference period of the last 4 months. Multiple regression was used to analyze the effects of explanatory variables on the frequency of alcohol, marijuana, and hard drug use. The findings show that age and family have significant interactive influences on alcohol and hard drug use. Age and church participation have significant interactive effects on marijuana and hard drug use. Age and peer influence interactively affect alcohol use. All significant independent and interactive effects were in the predicted direction. The interactive effect of age with other variables in predicting alcohol, marijuana, and drug use was not additive, but multiplicative. Although each of the various developmental theories emphasizes a different but overlapping set of variables, this article suggests that one common trait may be the interactive nature of age. The authors advise that since this is only one study, generalizations must wait for additional pertinent studies. 21 references, 5 tables, and 1 figure