It has been shown that the interplay of degenerative and inflammatory processes has a role in the etiology of osteoarthritis. The Renin-Angiotensin System (RAS) has reported in knee OA pathophysiology and ACE level influenced by ACE I/D polymorphism. This study aimed to examine the influence of the interaction between age and the DD genotype of the ACE gene on knee OA severity. A total of 80 knee OA patients (mean age 62.9 ± 8.84 years) were included in this cross-sectional study. The severity of knee OA was determined based on the Kellgren–Lawrence system. The Kolmogorov–Smirnov test, chi-square test, and logistic regression multivariate analysis were employed in statistical analysis. This study included 80 female subjects: 40 subjects with mild knee OA and 40 subjects with severe knee OA. The mean age of 63.7 ± 7.7 years was found for 34 subjects (42.5%) aged ≥65 years and 46 subjects (57.5%) <65 years. Based on the ROC curve, the cut-off age is 65 years. The DD gene ACE-1 genotype were not proven to increase the risk of severe knee OA. The DD gene ACE-1 genotype increases the risk of severe OA of the knee by 4 times in subjects aged ≥65 years compared to subjects <65 years. Our study found that the DD genotype ACE-1 did not increase the risk of severe knee OA. At age 65 years, the DD genotype ACE-1 enhances the risk factors for knee OA severity.