Understanding population dynamics is crucial to understanding current and future health care needs and designing systems to meet those needs. In this paper, we provide a methodological approach to investigate population dynamics in a system dynamics model configurable to initialise in dynamic equilibrium or disequilibrium. We then use the model to investigate how the current measured population compares to a population of the same size in equilibrium, and how a dependency ratio will change over time under different scenarios. We apply our approach to Singapore, which, like many other countries, has a rapidly increasing proportion of elderly in the population.