In a growing population, the younger population (or pre-reproductive individuals) size is larger than that of reproductive individuals. Such a population is represented by a triangular-shaped age pyramid whereas, the equal number of reproductive and pre-reproductive individuals represents a stable population, and the age pyramid is bell-shaped.
Less number of pre-reproductive individuals than reproductive individuals represents a declining population, and the age pyramid appears urn-shaped. A similar case is seen when reproductive individuals are less than post-reproductive individuals.​​​