COVID-19 vaccine recommendations are evolving in the United States. While older adults are most at risk of severe COVID-19 outcomes and therefore experience the greatest direct benefits of vaccination, limiting vaccination to only this age group could worsen outcomes in this higher-risk population. Here, we leveraged data from a statewide survey in Illinois to inform transmission models accounting for contact and vaccination rates across age. Simulating a single season of COVID-19 transmission, we compared deaths under existing vaccination coverage against counterfactual scenarios where individuals under 5 or under 65 were never vaccinated. We find substantial indirect vaccine impacts for older adults. Our results suggest that existing vaccination coverage among younger people is mitigating COVID-19 mortality for older populations. These findings can provide insights into the long-term consequences of deprioritizing young adults and children from vaccination campaigns, and suggest that a lack of vaccine-induced immunity may impact outcomes in other age groups. This underscores the importance of considering indirect vaccine impacts when developing policy.