Older age and comorbidities are risk factors for increased coronavirus disease 2019 (COVID-19) severity, but few studies have explored their interaction. This study aimed to assess the actual impacts of these factors on disease severity in COVID-19. The enrolled COVID-19 patients were divided into 4 age subgroups (<= 44, 45-59, 60-74, and >= 75 years). Logistic regression analysis was conducted to determine the association between comorbidities and disease severity; Kappa consistency test was implemented to verify the study results. Of the 1663 patients with COVID-19, 287 had severe disease. The disease severity was correlated with the age-adjusted Charlson Comorbidity Index in each age group. In the 4 subgroups, the odds ratio of age-adjusted Charlson Comorbidity Index declined with age. After removing age interference, diabetes and cardio-cerebrovascular diseases were the main risk factors for severe disease in patients aged <75 years, whereas only chronic lung disease was associated with disease severity in patients aged >= 75 years. When comorbidities alone were used to predict disease severity, only the predictions were consistent with real outcomes in patients aged >= 75 years, compared with the predictions of high-risk comorbidities mentioned in World Health Organization and Chinese guidelines (Kappa 0.106, P < .05). Although older age and comorbidities were risk factors for severe COVID-19, their effects on disease severity varied across age groups. Additionally, comorbidities had a greater impact on COVID-19 severity in younger patients.