Final answer:
Age is the demographic factor that classifies people based on life-stage. It is used by demographers to analyze populations and provides insights into factors such as birth rates, death rates, and life expectancy.
Step-by-step explanation:
The demographic factor that classifies people based on life-stage is age. Age is an important characteristic used by demographers to analyze populations. Age structure, which refers to the distribution of different age groups within a population, can provide insights into factors such as birth rates, death rates, and life expectancy.
For example, a population with a higher proportion of younger individuals may have higher birth rates and a growing population, while a population with a higher proportion of older individuals may have lower birth rates, higher death rates, and a declining population.