The age structure of breeding females of Hynobius nebulosus has not been studied sufficiently. We estimated the ages of 76 individuals from a population in Kyoto by using skeletochronology. The mean age and snout-vent length (SVL) of this population were 4.6 years and 55.7 mm, respectively. It was estimated that the youngest females breed two years post hatching at a mean SVL of 46.5 mm, but a larger number of individuals begins breeding at three years and a mean SVL of 52.2 mm. Because most males also start to breed at three years, there seems to be no gender difference in the timing of sexual maturation. The age of the oldest female was estimated to be 11.8 years. It is possible that the life history of H. nebulosus is characterized by early maturation and arrested growth, and short longevity.