As rural populations age at an accelerating pace, the role of public spaces in enhancing the quality of life and promoting social engagement among older adults has become increasingly important. However, a significant cognitive gap persists between the actual needs of elderly users and the intentions of designers, often resulting in suboptimal design outcomes and underutilized spaces. This study centers on the concept of user–designer cognitive synergy, aiming to establish a systematic framework to bridge this cognitive divide and improve the design quality of age-friendly rural public spaces. Employing Grounded Theory, the FKANO model, and the DEMATEL method, this study systematically elicited user needs, classified their attributes, and mapped causal relations to determine priority drivers. Applied in a representative rural case, the framework identified Environmental and Operations Management and Spatial Accessibility as the most critical needs, while Smart-Friendliness and Safety Organization were also shown to play significant roles. These findings directly informed targeted spatial strategies such as barrier-free circulation, robust nighttime safety systems, intergenerational hubs, and an operations backbone. Post-occupancy evaluation confirmed high satisfaction across safety, accessibility, functionality, social participation, and environmental comfort. The results demonstrate the framework’s effectiveness in translating complex needs into actionable design strategies, offering both theoretical insights and practical guidance for age-friendly rural public space development.