The proportion of older adults (OAs) is increasing throughout the globe. One of the most important needs of this age group is to maintain active social engagement, which can be achieved through smartphone applications offering active social engagement facilities. But OAs are often reluctant to use these apps, which leads to lower uptake over time. The underlying problem is pitfalls in guidance to develop smartphone applications for older adults. This leads to a lack of usage of the guidance by practitioners and hence the reflection of these guidelines on apps for older adults is limited. We have researched this problem space by undertaking a mixed-methods empirical study with 235 OAs. We elicited requirements that can be used in the design and development of smartphone applications for older adults. The contribution of this study is an evaluated classification of the derived recommendations, called ReDEAP (Recommendations for DEveloping smartphone applications for an aging Population). ReDEAP serves as a starting point for researchers and practitioners designing smartphone applications for older adults.