The time-scale of momentum effects
The larger the difference between the initial and final (stationary) age structure, the longer it will take for a population to become stationary. This phenomenon implies that populations with high growth rates take longer to 'settle down' because these populations tend to have a very young age structure. Low fertility populations, on the other hand, become stationary relatively quickly because they already have low growth rates and fairly old age structures. High mortality populations have a tendency to become stationary more quickly, because the high mortality rates quickly dilute the perturbations in the age structure caused by the imposition of the replacement level fertility regime. The age patterns of fertility and mortality are of secondary importance compared to the levels of fertility and mortality. This justifies the common practice of proportionally scaling up or down the observed age-specific fertility rates when determining replacement fertility i.e. 
The time that it takes for a population to become stationary is usually related to the amount of growth (positive or negative) experienced in the interim period. This assertion does not hold for populations with irregular population age structures at the outset. The case of the Most Developed Countries presented hereafter is a good example of such an exception.