Data from a longitudinal sample of adolescents from the United States were stratified by age and used to assess these potential differences. Precise measures of family structure were constructed to account for the various manifestations of family forms. Multivariate analyses show that a recent divorce attenuates attachment among younger adolescents and leads to less family involvement among older adolescents. Moreover, older adolescents from stepparent families are less attached to their families. Changes in these family-relationship variables influence associations with drug using peers, but these processes differ for the two age groups. Less family involvement and greater drug- using peer associations lead to greater marijuana use among younger adolescents. Only peer associations directly affect use among older adolescents. In accordance with past research, there is a marked transition in direct influence from families to peers as an individual passes through adolescence. This view, however, should be revised to take into account the effects of family relations on peers. These effects are significant and show that family relations can impede marijuana use indirectly, even among older adolescents. 1 table, 3 figures, and 64 references.