Human population, whether rising or declining, should be left as individual and parental decision and responsibility, not government decision and responsibility. Should there be population explosion or population control/depopulation, it should not be government-sponsored.
Thus, in ageing societies, instead of government giving subsidies to households for having new babies, government can reduce income taxes, relax rigid labor laws, so that people will have more money for their kids, or they can hire a nanny to help them raise kids.
In countries where thegrowth of population is flat or declining, there are two solutions: have more robots, or have more migrants. Welfarism is an ideological enemy of free migration.
Demography 21: China Overtaking the US' GDP Size in 2014?, May 02, 2014
Demography 22: Japan's Depopulation and Migration Policy, May 11, 2014
Demography 23: The UN, Depopulation and Climate, May 03, 2015
Demography 24, The Maddison Project data, August 13, 2015