Background: The differences in the characteristics and main causes of critical COVID-19 infection in non-elderly and elderly severe patients remain unknown. Methods: We included 273 adult patients with confirmed severe COVID-19 from Tongji Hospital, Wuhan, China from February 10 to March 8, 2020. Clinical characteristics and risk factors for outcomes were compared between the young and middle-aged and the elderly severe patients. Results: Hemoglobin, neutrophil percentage, inflammatory markers, hepatic, renal, and cardiovascular parameters differed between the non-elderly and elderly severe patients. In young and middle-aged patients, critical patients showed higher high-sensitivity C-reactive protein (hsCRP) during hospitalization than severe patients. However, in the elderly patients, critical patients showed decreased hsCRP during hospitalization and higher proBNP values. The hsCRP fluctuation and proBNP were independent risk factors for intensive care unit (ICU) admission in young and middle-aged severe patients (OR=1.068) and elderly severe patients (OR=1.026), respectively. Conclusion: The study revealed different potential causes of disease and predictive factors for non-elderly and elderly critical patients and treatment recommendations. Deterioration of inflammatory state was the main cause of ICU admission in young and middle-aged severe COVID-19 patients, while a decline in hsCRP was not associated with better outcomes in elderly severe patients, indicating the need for different treatments for non-elderly and elderly severe patients. Anti-inflammatory therapy with corticosteroids should be considered in the early disease stage among non-elderly severe patients, but cardiovascular protection plays a more important role in elderly severe patients.