Wayfinding in complex indoor environments is often challenging for older adults due to declines in navigational and spatial-cognition abilities. This paper introduces NavMarkAR, an augmented reality navigation system designed for smart-glasses to provide landmark-based guidance, aiming to enhance older adults' spatial navigation skills. This work addresses a significant gap in design research, with limited prior studies evaluating cognitive impacts of AR navigation systems. An initial usability test involved 6 participants, leading to prototype refinements, followed by a comprehensive study with 32 participants in a university setting. Results indicate improved wayfinding efficiency and cognitive map accuracy when using NavMarkAR. Future research will explore long-term cognitive skill retention with such navigational aids.