The few studies to date investigating age trends have yielded mixed findings. While studies among adults typically focus on clinical definitions of substance use disorders and MDD, studies of adolescents and young adults often define these constructs more broadly as substance use and depressive symptoms, as younger individuals may be in earlier stages of substance use and depression onset and not yet meet clinical definitions. Early studies found that substance use disorders were more strongly associated with depression for those over age 30 compared to those under age 30 (Grant, 1995), and that comorbid mental health problems and alcohol dependence were more common at older ages, whereas comorbidity with alcohol abuse was more common at younger ages (Grant and Harford, 1995). Fergusson et al. (2002) found that marijuana use is more strongly associated with suicidal ideation for adolescents compared to young adults, while Pedersen (2008) found that the association may be stronger for young adults. Poulin et al. (2005) found that the association between heavy episodic drinking (HED) and depression varied by age for females but not males. A recent
