Populations are a fundamental level of biological organization that poses major challenges for analysis. Individual traits that influence development, diapause, reproduction, aging, and lifespan interact in complex ways to determine birth and death. Birth and death drive population dynamics and determine whether a population survives or is doomed for extinction. However, we lack a deep understanding of the relationships between individual traits and population dynamics, a major challenge in the emerging field of ecology-development (eco-devo). Here we establish a laboratory ecosystem using the model organism C. elegans and a computational simulation that realistically models the laboratory ecosystem. We used these platforms to investigate the conditions that permit animals in the population to die of old age, a critical step in understanding the role of aging in population dynamics. Old age as a cause of death was influenced by three conditions: maximum lifespan, rate of adult culling, and progeny number/food stability. Remarkably, populations displayed a tipping point for aging as the primary cause of adult death. With high numbers of progeny, almost all adults in a population died young, whereas a slight decrease in progeny number caused a dramatic shift in the population, and almost all adults died of old age. The conditions defined here establish a conceptual framework for understanding why certain animals die of old age in the wild, such as mayflies and elephants.