Low- and middle-income countries are currently experiencing fertility declines at earlier developmental stages than historical precedents. However, the specific economic threshold at which fertility falls below the replacement level (2.1) remains under-analyzed. This study utilizes a comprehensive dataset to systematically visualize this threshold and quantify the economic discrepancy between historical and modern transitions. We demonstrate that today's developing economies reach the replacement level at significantly lower income levels than high-income economies did. This acceleration is driven by the rapid diffusion of health and education best practices and urbanization. Consequently, these nations face the dual challenge of "premature aging" and underdeveloped institutions. We conclude that sustainable development strategies must urgently shift from simple population control to building resilient social systems capable of withstanding these demographic headwinds.