Understanding the electroencephalography features of young and old patients treated with anesthetic drugs is important to allow accurate drug use in elderly patients. This study aimed to monitor the intracranial electroencephalography (in the cortex and hippocampus) in free-moving young and old mice under midazolam administration. Behavioral assessment revealed that compared with young mice, old mice had a longer immobility time with a similar midazolam dose. In both young and old mice, midazolam significantly suppressed the total, delta (0.5-4Hz), theta (4-8Hz), and alpha (8-12Hz) power, and thus induced an increase in the relative beta (12-30Hz) and gamma (30-140Hz) power. Age had a main effect on the gamma frequency; specifically, under normal conditions, old mice had a lower gamma power than young mice. After midazolam administration, the relative power of high gamma frequency (50-140Hz) remained lower in old mice than in young mice. Our findings suggest that a lower gamma power is indicative of an aging brain.