Age-specific changes in deer demographic parameters under food limitation are an important factor in predicting the post-population-crash dynamics of irruptive deer populations. To evaluate the differences in age-class-at-death between the initial irruption and post-population-crash phases of a sika deer (Cervus nippon) population, we analyzed a dataset of naturally dead carcasses from an introduced sika deer population on Nakanoshima Island, Japan. We predicted that the occurrence probability of fawn-class sika deer carcasses would increase in the post-population-crash phase and that the occurrence probability of old-class sika deer carcasses (≥ 10 years old) would decrease during this phase. Contrary to our expectations, the relative occurrence probabilities of four age classes [fawn, yearling, prime-aged (2–9 years old), and old] of sika deer carcasses did not differ between the initial irruption and post-population-crash phases. This could partially be a side-effect of classifying deer age into just four categories; slight changes within the age classes could not be detected because of this statistical limitation. Comparative studies are required to determine whether these results coincide with other irruptive deer populations.