Existing research has revealed key factors influencing mathematical flexibility, defined as the capacity to understand, generate, and apply a variety of strategies in solving mathematical problems. However, there is currently a lack of an integrated theoretical framework to systematically consolidate various sources of individual differences in mathematical flexibility. This issue impedes researchers' comprehensive understanding of the myriad factors influencing mathematical flexibility and weakens educators' ability to effectively intervene in students' learning. To address these challenges, we conducted a systematic review of various sources contributing to individual differences in mathematical flexibility, based on existing empirical research. We analyzed different explanations for mathematical flexibility differences in five aspects: demographic, cognitive, metacognitive, motivational, and emotional, leading to the development of the Individual Differences Model of Mathematical Flexibility (ID Model). Building on research gaps identified in this model, we further revealed potential new directions for future research. This study provides a valuable framework for mathematics education researchers to extend their theoretical research and offers novice instructors the opportunity to adapt practices for enhancing students' mathematical flexibility.