An analysis of the age structure of three water frog species (P. lessonae, P. ridibundus, P. esculentus) in the Middle Volga region of Russia was conducted. The skeletochronological method determined their maximum lifespans to be 7, 6, and 4 years, respectively. This study revealed specific characteristics of the age structure of Pelophylax frogs in relation to their taxonomic composition when cohabiting (population system type) and the degree of habitat transformation. Differences in body size and lifespan were observed among cryptic forms exhibiting mitochondrial DNA replacement and the presence of various types of heterozygous and homozygous alleles. Water frogs of the genus Pelophylax have a complex taxonomic structure, including hybrid species and morphologically cryptic forms, as well as massive mitochondrial DNA introgression. The aim of our paper was to analyze the age structure of three species of water frogs (P. lessonae, P. ridibundus, and P. esculentus) in the Middle Volga River region (European Russia). Based on the skeletochronological method, we determined their maximum lifespans of seven, six, and four years, respectively. The onset of sexual maturity in the marsh frogs occurred after two and four winterings in males and females (respectively), in the pond frog—one to two years, and in the edible frog—one to three years. This study revealed specific characteristics of the age structure of Pelophylax frogs in relation to their taxonomic composition when cohabiting (population system type) and the degree of habitat transformation. Differences in body size and lifespan were observed among cryptic forms exhibiting mitochondrial DNA replacement and the presence of various types of heterozygous and homozygous alleles.