Abstract: The determination of the individual age and ontogenetic stage (juvenile, subadult, adult) of fossil vertebrates is important for the initial determination of the taxonomic affiliation, as well as for further evolutionary and paleobiological interpretations. Determination of the individual age and ontogenetic stage (=relative age) is carried out by various methods, including paleohistological analysis. The study of thin sections of tetrapod bones allows us to assume how many years the animal lived (skeletochronological method) and to determine the ontogenetic stage according to a set of age-related histological markers: change in the type of bone matrix and vascularization, change in the distance between growth marks, formation of the external fundamental system (EFS), formation of the inner (endosteal) and outer (periosteal) circumferential layers (OCL, ICL), occurrences of secondary remodeling—Haversian substitution and formation of trabeculae. Depending on the phylogenetic position and biological peculiarities of the study group of tetrapods, the set of age “histological markers” may be different.