Stationary Population Dynamics Reveal a Structural Typology of Global Aging: A Binary Model Approach Across 195 Countries.

This study presents a unified structural typology of global population aging using a novel Binary Population Pair framework. Grounded in the Stationary Population Identity, the approach pairs each observed national population with its stationary counterpart to quantify deviations across three core dimensions: age structure, population momentum, and longevity gains. Leveraging new comparative metrics, including the Lifespan Parity Ratio, Stationarity Gap, Terminal Dependency Ratio, Survival Offset, and Net Structural Aging, the study classifies 195 countries into a five-stage demographic succession model, from Youth Dominance to Age Dominance, with a transitional Youth-Age Crossover at the midpoint. Results reveal a broad convergence toward structural stationarity by 2100, as countries transition from youthful to increasingly gerontic profiles. A post-successional condition, the "Demographic Vortex," is introduced to describe populations caught in a feedback loop of chronic low fertility and persistent structural aging. By integrating life lived and life left distributions, this framework captures aging as a directional, cumulative process. More fundamentally, it reconceptualizes aging not as a fixed trajectory toward senescence, but as a dynamic repositioning within an expanding and structurally shifting lifespan-transforming our understanding of what it means to age in the modern era.