In a multipolar world order, the demographic shift resulting from rapid population aging is affecting the fundamental unit of national power—the population—which, in turn, can influence a state’s perception of national security. This article examines the relationship between population aging and national security within the framework of “geriatric peace” by Haas (Int Secur 32(1):112–147, 2007). Utilizing panel data from 150 countries between 1992 and 2021, this study reveals two key findings. First, population aging leads to a relative increase in the priority given to social security over national security, resulting in a crowding-out effect on military spending. Second, the crowding-out effect of population aging on military spending is more pronounced in older countries compared to younger ones. To address potential endogeneity, this study employs instrumental variables (IV) estimation, treating population aging as an independent variable rather than a conditional one.