gradual, stage-based change of the organism expressed through quantitative and qualitative shifts. In a broad sense, it refers to the entire human lifespan from birth to death and includes not only progressive but also regressive changes. Periodization — the division of ontogenesis into distinct periods according to general developmental laws—constitutes a central problem field of developmental psychology. L. Vygotsky, in his work The Problem of Age (1932–1934), analyzed ontogenesis in childhood as a regular process of alternating stable and critical age periods. Individual development in adulthood represents a continuation of ontogenesis based on an inherent phylogenetic program. After early adolescence, maturation continues through stages of youth, adulthood, maturity, and old age, without rigid boundaries, depending on developmental regularities and involutional processes. Research demonstrates that maturation and involution are characterized by unevenness and heterochrony, which create diverse life trajectories ranging from premature aging to longevity.