BACKGROUND: Nonmedical prescription opioid use (NMPOU) is a serious public health problem in North America. At a population-level, previous research has identified differences in the prevalence and correlates of NMPOU among younger versus older age groups; however, less is known about age-related differences in NMPOU among people who use illegal drugs. METHODS: Data were collected between 2013 and 2015 from two linked prospective cohort studies in Vancouver, Canada: the At-Risk Youth Study (ARYS) and the Vancouver Injection Drug Users Study (VIDUS). Factors independently associated with NMPOU among younger (ARYS) and older (VIDUS) participants were examined separately using bivariate and multivariate generalized estimating equations. RESULTS: A total of 1162 participants were included. Among 405 eligible younger participants (Median age = 25; Inter-Quartile Range [IQR]: 22-28), 40% (n = 160) reported engaging in NMPOU at baseline; among 757 older participants (Median age = 48, IQR: 40-55), 35% (n = 262) reported engaging in NMPOU at baseline. In separate multivariate analyses of younger and older participants, NMPOU was positively and independently associated with heroin use (younger: Adjusted Odds Ratio [AOR] = 3.12, 95% Confidence Interval [CI]: 2.08-4.68; older: AOR = 2.79, 95% CI: 2.08-3.74), drug dealing (younger: AOR = 2.22, 95% CI: 1.58-3.13; older: AOR = 1.87, 95% CI: 1.40-2.49), and difficulty accessing services (younger: AOR = 1.47, 95% CI: 1.04-2.09; older: AOR = 1.74, 95% CI: 1.32-2.29). Among the youth cohort only, NMPOU was associated with younger age (AOR = 1.12, 95% CI: 1.05-1.19), crack use (AOR = 1.56, 95% CI: 1.06-2.30), and binge drug use (AOR = 1.41, 95% CI: 1.00-1.97); older participants who engaged in NMPOU were more likely to report crystal methamphetamine use (AOR = 1.97, 95% CI: 1.46-2.66), non-fatal overdose (AOR = 1.76, 95% CI: 1.20-2.60) and sex work (AOR = 1.49, 95% CI: 1.00-2.22). DISCUSSION: The prevalence of NMPOU is similar among younger and older people who use drugs, and independently associated with markers of vulnerability among both age groups. Adults who engage in NMPOU are at risk for non-fatal overdose, which highlights the need for youth and adult-specific strategies to address NMPOU that include better access to health and social services, as well as a range of addiction treatment options for opioid use. Findings also underscore the importance of improving pain treatment strategies tailored for PWUD.