Objectives <p>As individuals age, cognitive function declines at varying degrees and rates. Cognitive reserve (CR) is thought to explain, to some extent, the individual differences observed in cognitive decline. However, evidence on the relationship between CR and cognitive trajectories remains limited. This study aimed to construct a proxy indicator of CR from a life course perspective and explore its relationship with cognitive trajectories.</p> Methods <p>Data were obtained from 3,460 older adults aged 60 and above, drawn from the 2011–2020 China Health and Aging Longitudinal Study (CHARLS). Proxy indicators for CR in early-life were sourced from the Life course panel, while middle and later-life indicators were derived from baseline data collected in 2011. Structural equation modeling (SEM) was employed to construct overall CR scores. Cognitive function was assessed through memory, orientation, and executive function, using group-based trajectory modeling (GBTM) to track cognitive aging trajectories over time. Logistic regression analysis was subsequently used to examine the association between CR and cognition over a 10-year follow-up period.</p> Results <p>Early CR’s most significant proxy indicators were education, type of health insurance in middle-life, and intellectual activity in later-life. Based on the final inclusion of 3,460 participants, three cognition trajectory groups were identified using GBTM: “Low-Rapid Decline” (23.0%), “Moderate-Gradual Decline” (40.3%), and “High-Stable” (36.7%). After adjusting for sociodemographic, lifestyle, and health variables, older adults with higher CR was associated with increased odds of being in the “High-Stable” cognition group (OR=1.9, 95% CI 1.74–2.07).</p> Conclusion <p>The study reveals that higher CR is more likely to be associated with more favorable cognitive trajectories, highlighting the cumulative life-course impact of CR on cognitive aging and robust longitudinal validity of its proxy indicators for promoting healthy aging worldwide.</p>