question: BACKGROUND: The current coronavirus pandemic (COVID-19) was caused by severe acute respiratory syndrome virus 2 (SARS-CoV-2). COVID-19 is characterized by atypical pneumonia, mild colds, and more severe illnesses, such as severe acute respiratory distress, thrombosis, organ failure, and various secondary bacterial and fungal infections. Notably, the severity of COVID-19 in different age groups is not well known, and the validity of clinical laboratory data remains unclear. METHODS: In this retrospective cross-sectional study, we examined differential regulation of clinical, hematologic, and inflammatory biomarkers in COVID-19 patients. We divided 104 COVID-19 patients into five different groups according to age (0−17, 18−45, 46−65, 66−79, and >80 years). Baseline data (sex, comorbidities, intensive care admission, and medications), hematologic markers, liver, and renal function tests, coagulation, and inflammatory markers were examined in these groups. Receiver operator characteristic (ROC) analysis was used to determine the optimal threshold for predicting COVID-19 biological markers. RESULTS: We found that the highest percentage (45%) of COVID-19 patients was in the age group of 46−65 years. The hematologic parameters (WBC, HB, and PLT) were normal between the patient groups. The area under the curve in ROC analysis showed significant differences in the levels of creatine, GGT, BUN, CRP, D-dimer, ferritin, AST, and procalcitonin between the patients of age groups 46−65 and 66−79 years. Renal biomarkers were significantly high in most patients, regardless of age. In contrast, the liver biomarkers, did not differ significantly between patient groups. CONCLUSION: The main finding of our study is that laboratory parameters such as GGT, creatinine, BUN, CRP, procalcitonin, ferritin and D-dimer were differentially regulated in COVID -19 patients of different age groups. Importantly, these laboratory parameters may help as clinical predictors to assess the severity of the disease in the population. We conclude here that age is an important factor influencing COVID-19 severity.