The effect of aerobic exercise on physical function and mental health in various adult age groups (young-old, 65-74; old-old, 75-84; oldest-old, ≥ 85 years) is unclear. The aim of this study was to investigate the effects of the Kohzuki Exercise Program (KEP) on physical function and mental health in these age groups. The KEP consisted of 40-min supervised sessions 3 times per week for 6 months as follows: 5 min of warm-up, 30 min of lower limb aerobic exercise, and 5 min of cool-down. A total of 50 participants (22 young-old, 20 old-old, and 8 oldest-old) who participated in the KEP completed at least 88% of the sessions. In statistical analysis, 3 (group: oldest-old, old-old, young-old) × 2 (time: baseline and after 6 months) analyses of variance were used to determine if there were significant main and interaction effects. Significant interactions were probed using the post-hoc paired t test. The Short Physical Performance Battery (SPPB) score showed significant group × time interactions after 6 months (p = 0.031). In the post-hoc test, oldest-old (p < 0.001), old-old (p < 0.001), and young-old (p < 0.01) groups had significantly better physical function after 6 months. However, none of the mental health measures showed group × time interactions at 6-month. Our results suggest that a 6-month KEP led to improved physical function in oldest-old, old-old, and young-old adults. The KEP was effective for oldest-old adults in particular. The KEP exhibits good adherence, making it suitable for a wide age range in society.