The accuracy of near-infrared spectroscopy (NIRS) for predicting the chronological age of adults of the rice weevil, Sitophilus oryzae (L.); the lesser grain borer, Rhyzopertha dominica (F.); and the red flour beetle, Tribolium castaneum (Herbst), three pests of stored grain, was examined. NIRS-predicted age correlated well with actual age of these three species. Age predictions in S. oryzae by using the NIRS method are not dependent upon adult sex or temperatures to which adult weevils are exposed. Results indicated that water content decreased with increasing age in rice weevil adults, and excluding wavelengths at which water absorbs NIR radiation reduced the accuracy of correct classification. Additionally, removing cuticular lipids from insects resulted in a significant decrease in classification accuracy of weevils, indicating that these compounds may be partly responsible for the ability of NIRS to differentiate young from old beetles. NIRS is a nondestructive technique that can be used to age-grade large numbers of adult stored-product beetles, information that could help to increase the accuracy of population models for these pest species.