BACKGROUND: With the rapid growth of mobile technology, ensuring accessibility and usability for older adults has become a critical concern. This systematic review evaluates existing age-friendly mobile app design research, identifying key usability barriers and effective strategies for improving accessibility. METHODS: This study reviews English-language research (Published between Jun. 2014 and Mar. 2025) on mobile applications for adults aged 60 + , focusing on user-centered design, usability testing, and age-friendly adaptations. It compares different levels of accessibility features and evaluates their impact on usability, satisfaction, and engagement across various settings. Studies were retrieved from four online databases (PubMed, Scopus, Web of Science, and IEEE Xplore). RESULTS: This systematic review initially reviewed 1,556 records. From these, 132 articles met the inclusion criteria. The findings highlight several essential design elements, including simplified navigation, enlarged text and touch targets, voice interaction, and error-tolerant interfaces. Participatory design methods enhanced usability and satisfaction, demonstrating the importance of co-designing applications with older users. However, challenges like cognitive overload, lack of digital literacy, and accessibility barriers persist. CONCLUSION: The review emphasizes the need for future research on Artificial Intelligence-driven personalization, long-term usability studies, and culturally inclusive mobile applications. By integrating age-friendly design principles, developers can enhance digital inclusion, promote independence, and improve the overall well-being of older adults in an increasingly digital world.