The age structure of animal populations plays a pivotal role in understanding their demographic dynamics and reproductive potential. While carapace growth ring analysis is widely used to estimate turtle ages, environmental variability often causes irregular growth patterns which can compromise the accuracy of this method. Research grounded in accurately determined ages remains limited yet is essential, especially when addressing vulnerable populations. This study investigated body size distribution and melanism in a captive population of 131 endangered freshwater turtles, Mauremys reevesii, with precisely known ages. The mean ages were 9.4 ± 3.9 years (max 16) for females and 5.5 ± 1.8 years (max 12) for males, indicating a longer lifespan in females compared to males. Females were significantly larger than males, demonstrating female-biased sexual size dimorphism. Sex could be distinguished from age three onward. Melanism appeared only in males, first being observed at four years of age, and the progression followed the order of eyes, claws, skin, and shell. The stage of melanism in male M. reevesii was significantly influenced by age rather than body size. These findings provide the first detailed age-based dataset on growth and melanism in this critically endangered species. This offers a scientific basis for more accurate population analyses and the development of targeted conservation strategies, while helping to bridge the gap between wild and captive populations.