IMPORTANCE The US Food and Drug Administration (FDA) is increasing its pace of approvals for novel cancer therapeutics, including for immune checkpoint inhibitors of programmed cell death 1 protein (anti-PD-1 agents). However, little is known about how quickly anti-PD-1 agents reach eligible patients in practice or whether such patients differ from those studied in clinical trials that lead to FDA approval (pivotal clinical trials). OBJECTIVES To assess the speed with which anti-PD-1 agents reached eligible patients in practice and to compare the ages of patients treated in clinical practice with the ages of those treated in pivotal clinical trials. DESIGN, SETTING, AND PARTICIPANTS This retrospective cohort study, performed from January 1, 2011, through August 31, 2016, included patients from the Flatiron Health Network who were eligible for anti-PD-1 treatment of selected cancer types, which included melanoma, non-small cell lung cancer (NSCLC), and renal cell carcinoma (RCC). MAIN OUTCOMES AND MEASURES Cumulative proportions of eligible patients receiving anti-PD-1 treatment and their age distributions. RESULTS The study identified 3089 patients who were eligible for anti-PD-1 treatment (median age, 66 [interquartile range, 56-75] years for patients with melanoma, 66 [interquartile range, 58-72] years for patients with RCC, and 67 [interquartile range, 59-74] years for patients with NSCLC; 1742 male [56.4%] and 1347 [43.6%] female; 2066 [66.9%] white). Of these patients, 2123 (68.7%) received anti-PD-1 treatment, including 439 eligible patients with melanoma (79.1%), 1417 eligible patients with NSCLC (65.6%), and 267 eligible patients with RCC (71.2%). Within 4 months after FDA approval, greater than 60% of eligible patients in each cohort had received anti-PD-1 treatment. Overall, similar proportions of older and younger patients received anti-PD-1 treatment during the first 9 months after FDA approval. However, there were significant differences in age between clinical trial participants and patients receiving anti-PD-1 treatment in clinical practice, with more patients being older than 65 years in clinical practice (range, 327 of 1365 [60.6%] to 46 of 72 [63.9%]) than in pivotal clinical trials (range, 38 of 120 [31.7%] to 223 of 544 [41.0%]; all P < .001). CONCLUSIONS AND RELEVANCE Anti-PD-1 agents rapidly reached patients in clinical practice, and patients treated in clinical practice differed significantly from patients treated in pivotal clinical trials. Future actions are needed to ensure that rapid adoption occurs on the basis of representative trial evidence.