<p>Due to the popularity of video lectures, an increasing number of older adults are engaging with this form of learning. Furthermore, co-viewing has become a common type of social interaction. Despite the popularity of video lectures among older adults, there is limited understanding of how to effectively support their learning through this medium. The present study tested the interaction effects of co-viewer similarity (young vs. older co-viewer) and the type of learning content (declarative vs. procedural knowledge) in video lectures on older adults’ learning performance, motivation, cognitive load, and PFC activation as measured by fNIRS. Our results found that older adults showed better learning performance, greater motivation, and reduced cognitive load when co-viewing with an older viewer compared to a young one, regardless of the type of learning content. Interestingly, for declarative knowledge (i.e., Traditional Chinese Medicine, older adults exhibited greater activation in the FEFs, S1, and SMC regions when co-viewing with an older viewer compared with a young viewer. For procedural knowledge (i.e., digital technology), they exhibited less activation in the dlPFC and SMC regions when co-viewing with an older viewer compared with a young viewer. Our findings suggest that educators should consider the age of co-viewers when designing video lectures for older adults, irrespective of the type of knowledge being taught.</p>