In this data-rich but concise book, Harper examines the unprecedented global demographic shifts currently under way and considers what they portend in various parts of the world. Japan and Europe reached “maturity” at the end of the twentieth century, with more people over the age of 60 than under 15. The rest of Asia will reach that milestone in the middle of this century, as will the overall population of the world. By midcentury, Asia will account for 54 percent of the world’s population, down from around 60 percent today, and Europe’s share will decline from around 12 percent to around seven percent. Meanwhile, Africa will remain mostly young as its population more than doubles in the coming decades. Harper is less interested in the ramifications of these shifts for global politics than in identifying the shifts’ sources in particular countries and exploring their social effects. But her work provides a powerful reminder that debates over immigration, social welfare, and inequality will intensify in the decades ahead in a world increasingly divided between older, richer people and younger, poorer ones.