Courts are in broad agreement that juveniles—defined as people under 18-yearsold — are less culpable than adults and thus punish them differently. Indeed, few would disagree that the adult criminal system should apply only to adults—people “fully developed and mature.” If separating adults and juveniles based on culpability is the goal, it begs a simple question: should the split happen at age 18? Some U.S. institutions imply that they believe an 18-year-old lacks the requisite maturity to assume certain responsibilities, including the House of Representatives and car rental agencies, which permit participation at 25. Looking globally, important institutions like the United Nations mark the start of adulthood at age 25. These entities align with mounting evidence from brain development research suggesting that a rational brain fully forms around age 25.