Key PointsQuestionHow have rates of opioid agonist treatment (OAT) and opioid-related deaths in youths changed over the last 9 years and how does this compare to trends among adults?FindingsThis cross-sectional study of public health data for individuals aged 15 to 44 years in Ontario, Canada, found that rates of opioid-related deaths increased in both youths and adults between 2013 and 2021, yet rates of OAT among youths declined over this time despite rising rates of OAT among adults.MeaningThese results suggest that a better understanding of prevalence of opioid use disorder in youths and barriers to accessing OAT is needed.