This paper analyzes how and why household debt distribution by the householder age has changed over the past decade both in Korea and the US. Data shows that the proportion of household debt held by younger households has decreased, while that held by older households has increased. Empirical analysis shows that a change in the demographic distribution of householders is the main driving force that has shifted the household debt distribution. Given that demographic aging is an inevitable trend, the proportion of household debt held by older households is also expected to increase. Therefore, the Korean government must preemptively prepare for the household debt problem, especially for debt held by older households, by strengthening macro-prudential policies, preventing asset price deflation, restructuring household debt contract structures, and reforming labor market inflexibility.