Thomas Piketty’s Capital in the Twenty-First Century reshaped contemporary debates on inequality through the now-canonical condition r > g, where r denotes the net rate of return on capital and g the rate of aggregate income growth. While this condition captures an important structural tendency of capitalist economies, it abstracts from demographic mechanisms that fundamentally shape how wealth is accumulated, preserved, and transmitted across generations. In particular, mortality, survival, and age structure are treated largely as background variables rather than as active components of inequality dynamics. This chapter develops a mathematically explicit framework that integrates age-structured demographic dynamics—originating with Edmond Halley’s seventeenth-century life tables—into modern analyses of capital accumulation and inequality. Drawing on actuarial reasoning and population dynamics, mortality is modeled as an endogenous operator acting on wealth distributions rather than as a neutral shock. Differential survival across wealth strata generates selection effects, extends accumulation horizons for the wealthy, and alters both the timing and magnitude of intergenerational transfers. By reconnecting economic inequality to demographic structure, this chapter provides a rigorous mathematical foundation for analyzing long-run stability and sustainability in socioeconomic systems.