This paper investigates the dielectric properties of the epoxy-mica stator winding insulation in a 24-year-old synchronous generator, which exhibited no significant insulation defects during operation. According to Chinese standards, generators in service for 20 years should undergo aging assessment. However, this generator had not yet reached its scheduled major overhaul time, so the aging evaluation test was postponed by 4 years. The test results show that the generator’s maximum partial discharge (PD) is below 10,000 pC; the dielectric loss factor (tanδN) under rated voltage is less than 6%, with a tanδN increment (ΔtanδN) of less than 4%; and the capacitance increase rate (ΔC) is below 8%. All test parameters meet the requirements. A comprehensive analysis indicates that the insulation condition of this generator remains in good overall condition. This study provides a reference basis for postponing the aging assessment of epoxy-mica stator winding insulation in generators that have no major or above-grade insulation defects during operation.