Background: This cross-sectional epidemiological study aimed to compare the differences in physical fitness variables according to the skinfold thickness in the thigh area in adults and Korean older individuals. Methods: We analyzed data from the 2015 National Fitness Survey. A total of 4034 healthy adults (2442 men, 1592 women) with an average age of 38.19 \pm 12.41 years and 880 healthy older subjects (369 men, 511 women) with an average age of 72.32 \pm 5.49 years participated in this study. The skinfold thickness of the thigh was measured using a skinfold caliper (Dynatron, Dynatronics, USA). The participants underwent physical fitness tests, including the hand squeeze strength test, abdominal curl ups, standing double-leg long jump, 50 m shuttle run, sit and reach, and 20 m shuttle run for adult men and women. Older men and women performed the following tests: hand squeeze strength, abdominal curl ups, sit to stand test, single leg balance (open eyes), sit and reach, Apley scratch test for shoulder mobility, and 6-minute walk test. Independent t-tests and Pearson correlation analyses were used for the analysis. Results: Among the older men, significant differences were found in thigh skinfold thickness (t = –21.122, p 0.001), abdominal curl ups (t = 2.165, p = 0.031), and sit and reach in men (t = 2.609, p = 0.009), and thigh skinfold (t = –29.611, p 0.001), and Apley scratch test for shoulder mobility in women (t = –2.120, p = 0.034). There was a significant correlation between thigh skinfold thickness and physical activity (thigh skinfold thickness) in adult men (t = –54.202, p 0.001), nondominant hand squeeze strength (t = 2.632, p = 0.009), abdominal curl ups (t = 4.292, p 0.001), sit and reach (t = 3.063, p = 0.002), twenty meters shuttle run (t = 4.657, p 0.001). However, no significant differences were found in dominant hand squeeze strength, standing double leg long jump, or 50 m shuttle run in men (p > 0.05). In adult women, there was a significant correlation between thigh skinfold thickness and physical activity (thigh skinfold thickness (t = –49.405, p 0.001), dominant hand squeeze strength (t = 7.789, p 0.001), nondominant hand squeeze strength (t = 6.944, p 0.001), abdominal curl ups (t = 5.347, p 0.001), standing double leg long jump (t = 5.890, p 0.001), sit and reach (t = 5.384, p 0.001), twenty meter shuttle run (t = 5.223, p 0.001). However, no significant differences were found in fifty meter shuttle run in women (p > 0.05). Among older men, only single leg balance and sit and reach were correlated with thigh skinfold (single leg balance r = 0.169, p = 0.01; sit and reach r = –0.201, p = 0.001). In women, only abdominal curl ups, sit and reach, and Apley scratch test correlated with thigh skinfold (abdominal curl ups r = –0.088, p = 0.002; sit and reach r = –0.137, p = 0.002; Apley scratch test r = 0.090, p = 0.041). Conclusions: The effect of thigh skinfold thickness on the level of physical activity was more pronounced in adults than in older subjects. Our findings show that muscle strength and body fat in the lower extremities can affect overall muscle strength, endurance, and balance.