This book chapter is based on a Master's thesis that examines the relationships between relative age, mental toughness and perceived performance levels of young boxers. It also aims to understand the effects of individual differences in sports. In sports sciences, mental toughness is of critical importance in terms of athletes' ability to cope with difficulties and their capacity to sustain their performance. This study aims to contribute to the literature by analyzing how the mental toughness and perceived performance levels of young boxers are shaped by the effects of demographic and experiential factors. In this context, the role of the relative age effect on the sub-dimensions of mental toughness, namely confidence, control and continuity, was examined and associated with the athletes' performance perceptions. It is believed that the findings of the study will shed light on strategies aimed at optimizing the development and performance of young athletes in particular.