Global increases in life expectancy and the growing ubiquity of mobile technology have made smartphones essential tools for older adults. However, age-related declines in visual, cognitive, and psychomotor abilities often create significant usability barriers, preventing this demographic from fully benefiting from digital services. Although numerous secondary studies have addressed these challenges, the existing knowledge remains fragmented. This paper presents a tertiary study-a systematic review of systematic reviews-aimed at synthesizing design recommendations and identifying the application domains with the highest incidence of usability problems. Based on an analysis of eleven selected secondary studies, we provide a consolidated set of recommendations categorized into visual, cognitive, psychomotor, and general guidelines. Furthermore, our results reveal that health, education, and social applications present the most critical usability challenges for older adults. These findings offer researchers and developers a unified framework for creating more inclusive and accessible mobile interfaces.