To determine how climate factors influence age, body size and sexual size dimorphism (SSD) in the Mediterranean region, we generated data on age and body size of the European Treefrog, Hyla arborea, in three Turkish populations with a latitudinal gradient. We estimated age structure (total n = 154), using skeletochronology. Mean body size of both sexes was smaller in a southern population (Antalya) than in northern populations (Canakkale and Rize) with female-larger SSD in the northern populations. A positive correlation was found between age and body size in each sex of all the populations, save the Antalya females. Since amphibian growth is reduced after maturity but continues towards the asymptotic size, interpopulation size differences may partly be explained by differences in longevity with four years in Antalya and five years in the other two populations. Comparing age and body size in three Turkish populations with those in three different populations (Greece, Switzerland and Germany) from the literature, there was a trend of South-to-North increase in body size with increased latitude and decreased temperature and aridity. The same trend occurred also in age structure (e.g., age at maturity/first reproduction, longevity). These results suggest that a difference in age structure between populations is a main factor for the geographic variation in body size of this species.