The demographic transition from high to low levels of mortality and fertility brings about changes that stretch the “arc of life,” making each stage of life longer and creating new ones—a phenomenon we call “the demographic stretch.” This stretch can transform societal structure, for example, by extending childhood, shifting working ages up, delaying marriage and childbearing, improving women’s status and equity, and pushing the burden of chronic disease and disability to older ages. Global health efforts must address the resultant economic and social changes.