Introduction: The present study aimed to assess the dietary total antioxidant capacity and diversity among older and younger adults. Materials and Methods: In total, 38 older adults aged ≥ 65 years and 38 younger adults aged between 19 and 30 years were included in the study. The participants’ general characteristics, dietary habits, and three-day dietary intake records were assessed. The dietary total antioxidant capacity and diversity score were calculated from the dietary intake records. Moreover, the serum total antioxidant capacity and oxidant status were assessed using commercial kits. Results: The dietary diversity was greater among the younger adults than among the older adults (p < 0.05). According to the dietary total antioxidant capacity values calculated from various databases, in males, the lipophilic and total oxygen radical absorbance capacity values were significantly lower among the older adults than among the younger adults. In females, the hydrophilic and total oxygen radical absorbance capacity, total phenol, and ferric reducing antioxidant power values were significantly lower among the older adults than among the younger adults. The serum total oxidant status and oxidative stress index of the older adults were significantly higher than those of the younger adults (p < 0.05). Furthermore, the dietary diversity score and serum total antioxidant capacity were associated with the dietary total antioxidant capacity. Conclusion: With aging, the dietary total antioxidant capacity and diversity decrease, while the oxidative stress index increases. Therefore, the consumption of foods with high antioxidant capacity and dietary diversity should be encouraged among older adults.