Opioid agonist therapy (OAT) remains the first-line therapy for people with opioid use disorder. Whereas overdose rates among adolescents and young adults (AYAs) remain high, little is known about their access to OAT. Therefore, we sought to evaluate factors that shape access to OAT among AYAs aged 14 to 26 years. Data were derived from the At-Risk Youth Study, a prospective cohort study that involves street-involved AYAs who use illicit substances in Vancouver, Canada. Generalized estimating equations were used to identify factors associated with OAT enrollment from September 2005 to October 2021. A total of 759 AYAs reported at least weekly opioid or OAT use, with a median age of 23 years and 65.7% self-identifying as male. At baseline, 147 participants (19.4%) were on OAT, and another 199 (26.2%) initiated OAT during study follow-up (median number of follow-up visits, 5 [Q1-Q3, 2.5-8]). In a multivariable analysis, being <19 years old (adjusted odds ratio [AOR], 0.40; 95% confidence interval [CI], 0.23-0.71), Indigenous ancestry (OR, 0.72; 95% CI, 0.52-1.00), homelessness (AOR, 0.65; 95% CI, 0.54-0.77), drug dealing (AOR, 0.73; 95% CI, 0.61-0.87), daily opioid use (AOR, 0.47; 95% CI, 0.40-0.55), and nonfatal overdose (AOR, 0.73; 95% CI, 0.60-0.89) were negatively associated with OAT use. This study identified a low rate of OAT access among AYAs. Adolescents and young adults were less likely to be on OAT if they were <19 years old, Indigenous, and possessed certain risk markers. These findings highlight the need for mitigation strategies to facilitate OAT access for this population and for additional harm reduction measures to support AYAs who do not want to use OAT.