Today, the world has the largest generation of young people in history, with 3.6 billion people under the age of 30 worldwide. A population's age structure (the relative size of each age group) deeply affects development opportunities and plays a major role in security and governance challenges. In 2007, Population Action International (PAI) published The Shape of Things to Come: Why Age Structure Matters to a Safer, More Equitable World. Here, PAI updates and extends the analysis. Three case studies on Haiti, Yemen and Uganda examine the challenges specific to countries with very young age structures and recommend policy solutions.