Emigration Higher Same Lower 4. Life expectancy Higher Same Lower 5. Normal retirement age Higher Same Lower 6. The birth rate Higher Same Lower 7. Depending on how you answer, you will have dramatically different forecasts of life 10 years out.
Demographic change occurs slowly, so it is a powerful and accurate forecasting tool.
Nonetheless, if you are not aware of changing demographics, the results will be a surprise, a sneak attack on your society, markets, and public policy. Many forces impact on demographics. Inexpensive housing might encourage people to have more children, for example. And a growing population has huge impacts on society. Think of economic development and pollution.