As a large anadromous fish, the ability of Acipenser sinensis to adapt to seawater is a critical factor determining the success of its conservation and restocking efforts in marine environments. To investigate differences in seawater adaptability among juvenile Acipenser sinensis, this study used healthy 6-, 9-, and 12-month-old juvenile Acipenser sinensis as experimental subjects. These fish were subjected to seawater acclimatization and rearing, with freshwater-reared individuals serving as controls. Comparative analyses were conducted on growth performance, osmotic indicators, metabolic indicators, and genes related to osmotic regulation in gill tissue following seawater adaptation across different age groups. The results showed: (1) The feeding rate and survival rate of seawater-reared individuals increased significantly with age, and body condition score showed an overall upward trend, with 12-month-old individuals significantly higher than 6- and 9-month-old individuals. The feeding rate of seawater-reared individuals at all age stages was significantly lower than that of freshwater-reared individuals; the survival rate of 6- and 9-month-old seawater-reared individuals was significantly lower than that of freshwater-reared individuals, while there was no significant difference at 12 months of age; the body condition score of 6-month-old seawater individuals was significantly lower than that of freshwater individuals, while no significant difference was observed at 9 and 12 months of age. (2) The osmotic pressure and Cl^? concentration of 6-month-old seawater individuals exceeded normal ranges, but decreased significantly to normal levels with increasing age, whereas Na? concentration and gill filament NKA activity did not show significant changes with age. At all ages, the osmotic pressure and Na⁺ concentration of marine individuals were significantly higher than those of freshwater individuals, while there was no significant difference in gill filament NKA activity between marine and freshwater individuals at 6 months of age, but significant differences were observed at 9 and 12 months of age. (3) At 6 months of age, blood glucose, cholesterol, triglyceride, and total protein concentrations in seawater individuals were significantly higher than those in freshwater individuals. At 9 and 12 months of age, there were no significant differences in these indicators between seawater and freshwater individuals; however, the levels in seawater individuals at 9 and 12 months of age were significantly lower than those at 6 months of age. (4) The relative expression levels of CFTR-1 and NKCC1 in the gill filaments of seawater individuals showed a trend of first significantly decreasing and then significantly increasing with age, while ATP1a1 significantly decreased with age. The relative expression levels of CFTR-1, NKCC1, and ATP1a1 in the gill filaments of seawater individuals at all ages were significantly higher than those in freshwater individuals. The results indicate that the seawater adaptation ability of Acipenser sinensis is positively correlated with age (size). The osmotic regulation system of 6- and 9-month-old Acipenser sinensis was still immature; by 12 months of age, the system had become more complete and mature, with enhanced basal metabolic capacity providing sufficient energy support for ion transport and excretion, indicating that the organism had largely adapted to the seawater environment. This study provides a scientific basis for optimizing and selecting size specifications for the seawater conservation and restocking of Acipenser sinensis.