Variations in family size may significantly shape the adoption of preventive health behaviors, particularly among aging populations. Using data from the 2021 Chinese General Social Survey (CGSS), this study investigates the causal impact of the number of children on preventive health behaviors of older adults, focusing primarily on COVID-19 vaccination. To address the endogeneity issue, we employ an instrumental variable approach exploiting exogenous variations in policy exposure induced by China’s “Later, Longer, Fewer” (LLF) family planning campaign. Our results reveal that having more children reduces the likelihood of older adults receiving the vaccine. This negative impact is particularly pronounced among urban residents and in regions with lower pandemic severity. Mechanism analyses suggest a health-penalty pathway, where parents with more children experience poorer physical health, increasing their likelihood of medical ineligibility for vaccination. Crucially, we find that this negative effect extends beyond pandemic contexts to routine preventive behaviors, including influenza vaccinations and regular health examinations. Our findings highlight how historical fertility decisions shape health behaviors, providing novel insights into the determinants of preventive healthcare among aging populations.