Sensitivity to motion direction is affected by stimulus size, contrast (D. Tadin & J. S. Lappin, 2005; D. Tadin, J. S. Lappin, L. A. Gilroy, & R. Blake, 2003), and observer age (L. R. Betts, C. P. Taylor, A. B. Sekuler, & P. J. Bennett, 2005). Here, we investigated the effect of spatial frequency on motion discrimination and how sensitivity changes in older adulthood. We measured stimulus duration thresholds for younger (18-30 years) and older (60-75 years) observers using drifting Gabor gratings that differed in size, spatial frequency, and contrast. A simple model characterized age differences in the threshold-vs.-size functions. The model parameter fits were consistent with an age-related decrease in the strength of spatial suppression. The model also provided good fits to the thresholds when plotted as a function of the total stimulus contrast energy, which suggests that age-related changes in summation and suppression can be modeled as reduced sensitivity to contrast energy. The summation parameter scaled according to stimulus spatial frequency in younger observers only. Suppression strength decreased as a function of spatial frequency in both age groups. Our findings provide additional evidence for age-related changes in summation and suppression mechanisms and suggest that the total contrast energy in the stimulus plays an important role in determining sensitivity to motion direction in both younger and older adults.