Simple tools for extrapolations of human mortality in rich countries
  Suitable assumptions for the Gompertz mortality law take into account the break in the time development observed recently by Wilmoth et al. They show how a drastic reduction in the birth rate and improved living conditions lead to a drastic increase in the fraction of old people in the population, and how immigration of half a percent of the population per year can mostly stop this increase. 
Dietrich Stauffer