Background/objectives <p>Reference data for bioelectrical impedance analysis (BIA) parameters remain limited in youth, making sarcopenia assessment challenging. This study aimed to establish BIA reference values and determine sarcopenia cutoff points and prevalence in youth.</p> Subjects/methods <p>This cross-sectional study analyzed 1451 youth aged 10–25 years who underwent BIA using data from a nationwide survey. Reference values for body composition were established using the least mean squares method, which estimates age-specific percentiles. Sarcopenia was defined using skeletal muscle mass index (SMI), fat-free mass-to-fat ratio (FFM-MFR), and appendicular skeletal muscle mass-to-fat ratio (ASM-MFR) with age- and sex-specific cutoff values.</p> Results <p>Muscle-related parameters, including fat-free mass, fat-free mass index, ASM, and SMI, increased during puberty in both sexes, with a more pronounced increase in males, followed by a plateau or gradual increase after adolescence. Fat-related parameters, including fat mass, fat mass index, and percentage body fat, decreased until age 14 years in males before increasing, whereas in females, they increased until adolescence and declined after early adulthood. The prevalence of sarcopenia was 2.05% in males and 1.04% in females based on SMI, 5.21% in males and 6.38% in females based on FFM-MFR, and 5.06% in males and 5.79% in females based on ASM-MFR.</p> Conclusions <p>This study established BIA-based body composition reference values for youth using nationally representative data, identified age- and sex-specific sarcopenia cutoff points and prevalence estimates, and highlighted age- and sex-specific differences. These findings provide a valuable resource for the early identification and management of sarcopenia in youth.</p>