This volume examines changing boundaries between childhood and adulthood in British society and culture at the beginning of the twenty-first century where these age boundaries are widely debated, policed, and contested to investigate alternatives to conventional ideas of growing up. Building on observations, especially in children's literature criticism, that human growth is shaped by a grand narrative that privileges adulthood, and on terminologies of non-normative growth, particularly in queer theory, this monograph develops growing sideways as a concept that queers this grand narrative by destabilising childhood and adulthood, and the boundaries between them. The concept is refined through close readings of twenty-first century British children's literature, television series, film, and participatory events, troubling age boundaries via specific strategies in three conceptual areas: appearance, play, and space. Exploring power structures around age and gender, this monograph traces growing sideways as a distinct and important alternative discourse of human growth.