The strain of longer lives and low fertility, leading to fewer workers and more retirees, threatens to upend how societies are organized — around the notion that a surplus of young people will drive economies and help pay for the old. It may also require a reconceptualization of family and nation. Imagine entire regions where everyone is 70 or older. Imagine governments laying out huge bonuses for immigrants and mothers with lots of children. Imagine a gig economy filled with grandparents and Super Bowl ads promoting procreation... Many countries are beginning to accept the need to adapt, not just resist. South Korea is pushing for universities to merge. In Japan, where adult diapers now outsell ones for babies, municipalities have been consolidated as towns age and shrink. In Sweden, some cities have shifted resources from schools to elder care. And almost everywhere, older people are being asked to keep working. Germany, which previously raised its retirement age to 67, is now considering a bump to 69. Going further than many other nations, Germany has also worked through a program of urban contraction: Demolitions have removed around 330,000 units from the housing stock since 2002... The challenges ahead are still a cul-de-sac — no country with a serious slowdown in population growth has managed to increase its fertility rate much beyond the minor uptick that Germany accomplished. There is little sign of wage growth in shrinking countries, and there is no guarantee that a smaller population means less stress on the environment.