Diabetes is associated with severe COVID-19 and mortality. The aim of the present study was to determine the effect of age on the association between diabetes and mortality in patients with laboratory-confirmed COVID-19 in Mexico. This retrospective cohort study involved patients aged 20 years or older with symptoms of viral respiratory disease who were screened for SARS-CoV-2 infection across the System of Epidemiological Surveillance of Viral Respiratory Disease in Mexico from January 1 through November 4, 2020. Cox proportional-hazard regression was used to calculate the hazard ratio for 28-day mortality and its 95% confidence interval (CI). Among 757,210 patients with COVID-19 (outpatients and inpatients), 120,476 (16%) had diabetes and 80,616 died. Among 878,840 patients without COVID-19 (those who tested negative for SARS-CoV-2 infection), 88,235 (10.0%) had diabetes and 20,134 died. Among patients with COVID-19, diabetes was associated with a hazard ratio for death of 1.49 (95% CI 1.47-1.52), adjusting for age, sex, smoking habit, obesity, hypertension, immunodeficiency, and cardiovascular, pulmonary, and chronic renal disease. The strength of the association decreased with age (trend test: P = 0.004). For example, the adjusted hazard ratio for death was 3.12 (95% CI 2.86-3.40) for patients 20-39 years of age; in contrast, the adjusted hazard ratio of death for patients 80 years of age or older was 1.11 (95% CI 1.06-1.16). The adjusted hazard ratios were 1.66 (95% CI 1.58-1.74) in outpatients and 1.14 (95% CI 1.12-1.16) in inpatients. In hospitalized patients 80 years of age or older, no association was observed between diabetes and COVID-19-related mortality (adjusted hazard ratio: 1.03; 95% CI 0.98-1.08). Among patients without COVID-19, the adjusted hazard ratio for death was 1.78 (95% CI 1.73-1.84). In conclusion, in adult patients with COVID-19 in Mexico, the risk of death associated with diabetes decreased with age. No association between diabetes and mortality was observed among inpatients 80 years of age or older. Our findings should be verified in other populations.