Understanding the risks associated with opioid prescription in adolescents is critical for informing opioid policy, but the risks are challenging to quantify given the lack of randomized trial data. Using a regression discontinuity design, we exploited a discontinuous increase in opioid prescribing in the emergency department (ED) when adolescents transition from "child" to "adult" at age eighteen to estimate the effect of an ED opioid prescription on subsequent opioid-related adverse events. We found that adolescent patients just over age eighteen were similar to those just under age eighteen, but they were 9.7 percent more likely to be prescribed an opioid and 12.6 percent more likely to have an adverse opioid-related event, defined as overdose, diagnosis of opioid use disorder, or long-term opioid use, within one year. We estimated a 14.1 percent increased risk for an adverse outcome when "adults" just over age eighteen were prescribed opioids that would not have been prescribed if they were just under age eighteen and considered "children." Our results suggest that differences in care provided in pediatric versus adult care settings may be important to understanding prescribers' roles in the opioid epidemic.