Understanding the relationships among leaf and size traits and environment factors is essential for understanding and predicting the dynamics and ecophysiological processes of moso bamboo (Phyllostachys edulis) forests across their distribution range. Here, we evaluated these relationships at six sites within the moso bamboo distribution area in China. Specifically, we collected climate (mean annual temperature and precipitation) and soil physicochemical data from the sites, and we measured 14 leaf traits and 2 size traits (height and diameter) of both new and old moso bamboos and evaluated the relationship among environment factors, size traits and leaf traits. Climate factors had significant effects on the leaf traits of new bamboos but on those of old bamboos. For instance, mean annual precipitation (MAP) was the main driver of leaf-trait variation in new bamboos but not in old bamboos. Specifically, MAP was significantly negatively correlated with leaf mass (LM), leaf dry matter content, single leaf area (LA), and leaf width, but positively correlated with specific leaf area and the leaf length to width ratio. Moreover, potential evapotranspiration and solar radiation (SR) significantly affected the leaf nutrient traits of new bamboos. Mean annual temperature and actual evapotranspiration were significantly positively correlated with bamboo height. Bamboo diameter at breast height (DBH) had more explanatory power with respect to leaf traits than plant height, and LA, leaf length and width, and LM all decreased with increasing DBH in new bamboos. A structural equation model showed that soil physicochemical traits significantly affected leaf traits, but not size traits, of new bamboos. Overall, our study indicated an age-dependent response of bamboo leaf traits to environmental factors, although bamboos do not have secondary growth. These results suggest that future environmental changes, including climate change, will lead to age-related responses of bamboo leaf traits, which may further result in changes in stand structure and productivity of moso bamboo forests.