Several studies used age at onset of alcohol dependence as a putative biological marker (Apprey et al., 2005; Le Strat et al., 2008a; Tayo et al., 2005; Zhong and Zhang, 2005). Particularly, age at onset could be genetically mediated, since genetic factors account for 49% of the variation in the age at onset of alcohol dependence (Liu et al., 2004). Moreover, the relationships between alcohol dependence and other addictive disorders are complex and involve several biological and clinical interactions (Funk et al., 2006; Ribeiro-Carvalho et al., 2008). Addictive disorder probably have in common neurobiological pathway (Ribeiro-Carvalho et al., 2008), and share part of the involved genetic and environmental influences to initiation (Xian et al., 2008), the development of a dependence (Kendler et al., 2008; Le Strat et al., 2008b) as well as the cessation of the disorders (McKee et al., 2008). Accordingly, the consideration of a more genetically homogenous subgroup of patients defined by the age at onset of alcohol dependence earlier than 22 years old, and characterized by a higher rate of addictive comorbidities could be meaningful.
