Population aging has become a salient demographic phenomenon in China, thus causing social and economic concerns. However, little is known about the impact of population aging on the environment. The available literature mainly emphasizes decreases in energy consumption or carbon emissions caused by population aging, conveying limited information about the impact of population aging on household carbon emissions. This paper explores the impact of population aging on household carbon emissions in both urban and rural areas of China, using panel data of 30 provinces over 1997–2017. The relationship between population aging and household carbon emissions is investigated with a threshold regression model and a two-stage instrumental variable regression model. The results show that urban population aging increases urban household carbon emissions at levels both below and above 0.083, while this positive impact is comparatively smaller when urban population aging is above 0.083. However, at the national level, rural population aging has no significant impact on rural household carbon emissions at the level below 0.066, while rural population aging significantly increases rural household carbon emissions at the level above 0.066. Rural population aging has a significant positive effect on the increase in household carbon emissions in northern heating areas, when rural population aging is both below and above 0.077. The analysis of the underlying mechanism identifies consumption structure and consumption level as intermediary factors that affect the nonlinear relationship between urban population aging and urban household carbon emissions. Furthermore, the income level also affects the nonlinear relationship between rural population aging and rural household carbon emissions.