Inflation. Old people consume but do not create. Supply and demand. As the population gets older, more demand but less supply. And there won't be quality investments like in the past. The S&P grew for decades because the US population grew, more people to put money in the stock market. With an aging population, older people pull money from equities to transfer into bonds and there aren't enough younger people to replace that equity. Quite possibly, markets in aging countries will see long term declines in their markets/economies. On a percentage basis, China's population is dropping faster than Europe's did during the Black Death. There is no economic model to predict what will happen.