Background <p>Evidence linking sarcopenia to cognitive outcomes is largely cross-sectional; longitudinal studies are limited, and whether biological age acceleration modifies this relationship remains unclear.</p> Methods <p>Data were obtained from the China Health and Retirement Longitudinal Study (CHARLS) spanning 2011–2015. Sarcopenia was identified based on three indicators: appendicular skeletal muscle mass, muscle strength, and physical performance. Cognitive trajectories were evaluated using standardized z-scores for global cognition and individual domains. Biological age was calculated using chronological age, creatinine, glucose, and C-reactive protein. To assess longitudinal associations, linear mixed-effects models with random intercepts and slopes were employed. We further tested for interactions and combined effects involving biological age acceleration.</p> Results <p>A total of 4,376 participants (mean age: 58.2 years; 52.9% female) were included. Sarcopenia was significantly associated with accelerated decline in three cognitive domains and global cognition [<i>β</i> = -0.11, 95% CI: -0.16 to -0.06]. Stratified analyses further indicated an interaction with biological aging, as the adverse impact of sarcopenia on cognition was more pronounced among participants with accelerated biological aging, particularly in memory and global cognition. In addition, when sarcopenia and accelerated biological aging coexisted, the combined effect corresponded to the greatest cognitive decline [temporal orientation: <i>β</i> = -0.17, 95% CI: -0.27 to -0.07; memory function: <i>β</i> = -0.16, 95% CI: -0.27 to -0.06; global cognition: <i>β</i> = -0.14, 95% CI: -0.22 to -0.06].</p> Conclusions <p>Sarcopenia is associated with a faster rate of cognitive decline, particularly among individuals with accelerated biological aging. These findings underscore the need to integrate sarcopenia management with strategies targeting biological aging to mitigate cognitive decline in older populations.</p>