This chapter explores what it means for a person to show maturity as a leader. We describe the defining characteristics of mature leadership and the developmental stages required to demonstrate it. After providing an overview of psychological, developmental, and spiritual definitions of human maturity, we propose two central defining characteristics of mature leadership. We argue that rites of passage provide essential fuel for growth and maturity in all human cultures. Drawing from theory and research in heroism science, we identify heroic transforming leadership as the pinnacle of mature leadership. We also explore the Jungian archetype of the puer aeternus, or “eternal boy,” referring to the immaturity that characterizes leaders who are stuck at early stages of social, psychological, and spiritual development. We show how Donald Trump’s behavior as a leader represents a striking instantiation of the puer aeternus, and we discuss implications for mature leadership.