Preweanling, postadolescent, and post-middle-aged CBA/J mice received injections of kanamycin sulfate for two weeks, followed by electrocochleographic (isoelectric auditory nerve-evoked potential thresholds from 2,000 to 64,000 Hz) and histocochleographic (hair cell counts from cochlea whole-mount preparations) analysis. The preweanling mice had structural and functional losses from all portions of the cochlea, while the postadolescent mice had no structural and minimal functional loss. The oldest mice had basal inner and outer hair cell loss and elevated high frequency thresholds. This is an experimental validation of the clinical observation that infants are most susceptible to aminoglycosidic ototoxicity. It also support the suspicion that older persons are also susceptible to ototoxicity and provides the first animal model for experimentally investigating these age-dependent effects.