The expansion of Moso bamboo forests in Japan might change transpiration and therefore reduce the availability of water resources. Moso bamboo stands are often composed of culms with various ages and older culms may have lower sap flux density (F_d), which may in turn affect individual culm transpiration (Q_t), probably because vascular bundles do not regenerate after sprouting. Information related to the differences of F_d and Q_t between younger and older culms would be important for (i) understanding the effects of culm age structure changes on stand-scale transpiration (E_C), and (ii) developing sampling strategies for E_C estimates in Moso bamboo forests. We conducted sap flux measurements for 15 individuals from four culm age classes in a managed Moso bamboo forest in Kameoka, Kyoto, Japan. Differences in F_d were not significant among the four culm age classes with almost the same stem diameter at breast height (DBH). Q_t was related to DBH across four age classes, indicating that culm age had no apparent effect on Q_t in the forest. Our results suggest the effects of culm age structure changes on E_C are small, and contribute to development of sampling strategy without considering culm age structure for E_C estimates at this site.