This study provides a descriptive resource on long-term trends in the absolute and relative competitive levels of Japanese swimmers by age-group class from 1991 to 2024. Finals results were compiled from the World Aquatics Championships and Olympic Games (World), the Japan Swimming Championships (Japan; senior elite), and the JOC Junior Olympic Cup (age-group junior class B, 11–12 years; C, 13–14 years; D_CS, 15–18 years). The overall time index (OTI), the overall world ratio (OWR), and event-specific world ratios were analyzed using linear regression with HAC standard errors and BH-FDR correction. Indicators reflect the level of finalists rather than the international competitiveness of individual representatives. OTI decreased significantly in all classes for both sexes. OWR trends differed by sex: women's B and C increased significantly (relative decline), while men's C and Japan decreased (relative improvement). The 100 m backstroke deviated from overall trends.