It has been reported that the spontaneous firing of striatal cholinergic interneurons (ChINs) increases with postnatal development. However, changes in spontaneous firing frequency and firing properties during aging has not been investigated yet. In this study, cell-attached and whole-cell patch-clamp studies were carried out to investigate changes in firing properties of ChINs during aging. Brain slices were prepared from 2–3-month-old, 11–12-month-old and 24-month-old mice of either sex. Frequencies of spontaneous firing at 2–3-month-old, 11–12-month-old and 24-month-old were 4.55 ± 1.01 Hz (n = 18), 8.73 ± 2.28 Hz (n = 8) and 2.88 ± 0.99 Hz (n = 11), respectively. Firing frequency at 24 month of age was significantly lower than that of 11–12-month-old (p = 0.028). Moreover, the voltage sag induced by hyperpolarizing current injection was recorded. Notably, the sag ratio at 24-month-old was smaller (1.1 ± 0.02, n = 6) than that observed at 2–3-month-old (1.2 ± 0.01, n = 7, P = 0.04) and 11–12-month-old (1.2 ± 0.02, n = 5, P = 0.01), which implies a decline in the h-current responsible for the voltage sag during the aging. Considering the established significance of the h-current in governing the spontaneous firing of ChINs, these findings suggest that the reduction in the h current may underlie the decrease in firing activity.