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Background/Objectives: Dietary habits are important modifiable factors influencing survival among cancer patients. The dietary quality among cancer survivors may differ from those of the general population and may vary according to age at cancer diagnosis. This study aimed to compare dietary quality between cancer survivors and the general population and to examine whether age at diagnosis is associated with dietary quality. Methods: This retrospective cross-sectional study used data from 8706 adults aged ≥ 30 years (641 cancer survivors and 8065 controls) from the 2019-2021 Korea National Health and Nutrition Examination Survey. Dietary quality was assessed using the Diet Quality Index-International (DQI-I; range 0-100). Survey-weighted multiple linear regression models were used to compare DQI-I scores between cancer survivors and controls. Subgroup analyses were stratified by age at diagnosis, and quadratic age terms were included to assess nonlinear associations. All analyses accounted for the complex survey design. Results: Cancer survivors had significantly higher mean DQI-I scores than controls (69.1 ± 0.4 vs. 66.1 ± 0.2; p < 0.001). Among survivors diagnosed before age 50, dietary quality was significantly higher in those currently under 65 years than in controls (mean difference +3.02, 95% CI 1.44-4.60), but notably lower in those aged ≥ 65 years (-3.18, 95% CI -6.16 to -0.20). In contrast, survivors diagnosed at age ≥ 50 consistently showed higher dietary quality than controls across all age groups (+3.76, 95% CI 2.83-4.68). Conclusions: While cancer survivors generally exhibit better dietary quality than the general population, this positive trend was not observed among younger-onset survivors in older age groups. These findings suggest that age at cancer diagnosis may be associated with dietary quality and highlight the need for sustained, age-specific nutritional support strategies in cancer survivorship.