Extracellular recordings were obtained from inferior colliculus (IC) neurons in young (2-month) and middle-aged (7-month; 12- to 13-month) C57BL/6J mice in response to contralateral tone and noise stimuli. An age-related progressive loss of spiral ganglion cells, most pronounced near the cochlear base, was observed in the mice, accompanied by severe high frequency hearing loss manifested as elevation of neuronal thresholds, especially in the ventromedial half of the IC. There was a small age-related increase (2% to 11%) in 'sluggish' neurons (auditory, but poorly driven by sound); however, most neurons were well-driven by suprathreshold stimuli. Nine response types were derived from post-stimulus time histograms; they were found in all age groups with little difference in their relative incidence. The percentage of neurons that were spontaneously active increased with age in the central nucleus but not in other subnuclei. Parameters of response areas (range, upper frequency range, best frequency, and rate-best frequency) showed pronounced age differences in the ventromedial half of the IC and minimal differences in the dorsolateral half of the IC. The percentage of neurons with nonmonotonic rate-level functions decreased with age, especially in the IC dorsal cortex.