As some of us know far too well, forecasting the future with any precision is extremely difficult. There’s at least one exception to the rule, though. Population trends show themselves decades and even centuries in advance. If we know how many people were born in a given year, we can extrapolate what the population will look like far in the future.
On the other hand, demographic forecasting still requires assumptions. At what age will people start having children, and how many will they have? How will new medical advances affect life spans?