A recently published study by Behrendt et al. (2012) examined the association between age at first alcohol and tobacco use, and cannabis use in early adulthood. They found that those individuals who began drinking alcohol at a younger age were more likely to smoke cannabis in early adulthood. Further, individuals who began smoking tobacco at a younger age were more likely to begin smoking cannabis at a young age as well. These findings suggest that the early onset of both alcohol and tobacco use have direct implications for cannabis use later in life.
Behrendt S, Beesdo-Baum K, Hofler M, Perkonigg A, Buhringer G, Lieb R et al. The relevance of age at first alcohol and nicotine use for initiation of cannabis use and progression to cannabis use disorders. Drug and Alcohol Dependence 2012, 123(1-3):48-56 (abstract)