Determining the age of any species allows it to be analyzed from the ontogenetic, demographic, and ecological perspectives. In the present study, we tested the hypothesis that the age structure of congener species (Lacerta media and Lacerta viridis) with the same ecological niche may vary in different areas. In this context, we applied skeletochronology method to reveal various demographic parameters, such as age structure, longevity, age at sexual maturity, growth rate, survival rate, adult life expectancy, and the relationship between age and body size in the green lizard, L. viridis, and the medium lizard, L. media. In L. media and L. viridis, the maximum lifespan was 10 and 8 years, respectively. The mean age and body size of females were significantly greater than those of males in L. media. However, in the examined L. viridis population, no appreciable variation in mean age or body size was found to exist between the sexes. It was estimated that the green lizards reach maturity at the age of 2 or 3 years. However, the L. media reached sexual maturity approximately 1 year later than the congener. The body size markedly increased with age in males for both studied populations. However, in females, body size positively increased with age only in L. media. The approach of skeletochronology that we utilized in this study to assess age structure makes it simple to gather a variety of time-dependent ecological data for such ectothermic species.