Proposals for expanding Medicare insurance coverage to uninsured Americans approaching the Medicare eligibility age of sixty-five has been the subject of intense debate. We undertook this study to assess cancer survival differences between uninsured patients younger than age sixty-five and older Medicare beneficiaries by using data from the National Cancer Database from the period 2004-16. The main outcomes were survival at one, two, and five years for sixteen cancer types in 1,206,821 patients. We found that uninsured patients ages 60-64 were nearly twice as likely to present with late-stage disease and were significantly less likely to receive surgery, chemotherapy, or radiotherapy than Medicare beneficiaries ages 66-69, despite lower comorbidity among younger patients. Compared with older Medicare patients, younger uninsured patients had strikingly lower five-year survival across cancer types. For instance, five-year survival in younger uninsured patients with late-stage breast or prostate cancer was 5-17 percent lower than that among older Medicare patients. We conclude that survival after a diagnosis of cancer is considerably lower in younger uninsured patients than in older Medicare patients. Expanding comprehensive health insurance coverage to people approaching Medicare age eligibility may improve cancer outcomes in the US.