Adolescence represents a critical period of physical, cognitive, and emotional maturation, during which regular physical activity plays an essential role in improving health outcomes. This review outlines the effects of exercise on bone density, muscle mass, cardiovascular function, and metabolic health in adolescents. In addition, it looks into the cognitive and psychological benefits of physical activity, including enhancements in brain function, academic achievement, and emotional well-being. The molecular mechanisms underlying these adaptations—such as changes in mitochondrial function, neuroplasticity, and myokine release—are discussed to provide a more extensive understanding of the biological processes influenced by exercise. Given the increasing prevalence of sedentary behaviors in childhood, this review highlights the urgent need to integrate organized physical activity programs into school and community environments to encourage lifelong healthy habits. Future research should aim to refine exercise prescriptions adjusted to adolescent development and overall well-being.