Gait speed is now recognized as an important clinical tool in the older adult population. However, fast gait speed appears to better reflect lower-extremity muscle performance and declines more rapidly in advanced age than comfortable gait speed. We examined the ability of leg lean mass and absolute and rapid strength characteristics to predict fast gait speed in younger versus older adults. Twenty-four younger (12 men, 12 women; age = 22 ± 3 years) and 22 older (11 men, 11 women; age = 72 ± 6 years) adults participated in the study. Fast gait speed was assessed at 10- and 400-m distances. Leg lean mass was quantified with dual X-ray absorptiometry. Isometric squat peak torque and the rate of torque development at 200 ms (RTD200) were tested with a 120° knee joint angle. Statistical analyses included independent samples t tests, partial correlations, and stepwise regression. Large differences between age groups were observed for peak torque, RTD200, and gait speed (P ≤ 0.006, d ≥ 0.79), whereas the difference in leg lean mass was small (P = 0.246, d = 0.35). In older adults only, the partial correlation for 400-m speed versus RTD200 was significant (r = 0.463, P = 0.040). Stepwise regression revealed that peak torque was a significant predictor of 10-m (R2 = 0.257, P = 0.016) and 400-m (R2 = 0.239, P = 0.021) gait speed in older adults. As adults age, lower-extremity, multi-joint muscle strength becomes increasingly important in regulating fast gait speed, whereas lean mass is not predictive.