The interrelationship between muscle strength, motor unit (MU) number, and age is poorly understood, and in this study we sought to determine whether age-related differences in muscle strength are moderated by estimates of functioning MU number and size. Eighteen older adults (OA; 67 ± 1.20 years) and 24 young adults (YA; 22 ± 0.74 years) participated in this study. Maximum voluntary pinch–grip strength of the nondominant hand was determined and estimates of MU number were obtained from the abductor pollicis brevis muscle using the noninvasive motor unit number index (MUNIX) technique. The MUNIX technique was also utilized to derive a motor unit size index (MUSIX). An analysis of covariance (Age Group × MUNIX or MUSIX) was used to test heterogeneity of regression slopes, with body mass and gender serving as covariates. We observed that the slope of pinch–grip strength on the estimated number of MUs between YA and OA differed, indicated by an Age Group × MUNIX interaction (p = 0.04). Specifically, after controlling for the effect of body mass and gender, the slope in OA was significantly positive (0.14 ± 0.06 N/MUs, p = 0.03), whereas no such relationship was found in YA (−0.08 ± 0.09 N/MUs, p = 0.35). A significant Age Group × MUSIX interaction was also observed for strength (p < 0.01). In contrast to MUNIX, the slope in younger adults was significantly positive (0.48 ± 0.11 N/μV, p < 0.01), whereas no such relationship was found in older adults (−0.30 ± 0.22 N/μV, p = 0.18). These findings indicate that there is an interrelationship between muscle strength, MU numbers, and aging, which suggests that a portion of muscle weakness in seniors may be attributable to the loss of functioning motor units.