The distinct abilities of older adults to interact with touchscreen devices have motivated a wide range of contributions in the form of design guidelines, which aim at informing the design for the aging population. However, despite the growing effort by the research community, many challenges still remain in translating these research findings into actionable design guidelines, with reports hinting scant adoption or implementation issues, which ultimately hurt the development of more accessible interactive systems. In this systematic literature review, we look at the research-derived design guidelines that set the foundation for design guideline compilations and standards, analyzing the aforementioned issues from the perspective of experts trying to discover, classify, and evaluate the work on the area of touchscreen design guidelines for older adults. The review analyses 52 research articles resulting in 434 research-derived design guidelines for touchscreen applications. These guidelines are analyzed using a taxonomy that considered the older adults ability evolution and the design aspects that are the target of the recommendations. The results point to the use of different definition of older adults, which go as early as 55+, with the design of displays and interaction styles to accommodate to vision and dexterity declines as the most prominent areas of research. However, proposed guidelines and recommendations were validated in only 15% of articles analyzed. The analysis also revealed that identifying guidelines and characterizing their focus in terms of ability declines and design aspects addressed is a demanding activity and prone to error, given the quality of reporting and details offered in research articles.