The capitalist economy is driven by GROWTH. As the population ages, they consume more resources than they create. With fewer younger workers contributing taxes and resources the elderly are requiring, programs for the aging population will be difficult to maintain.
Unless there is mass migration to increase the working population, the economy will start to collapse. Of course, some economies are already in trouble because of declining populations - See Russia, Hungary. The USA will soon join them if they maintain their current attitudes towards immigration.
"The attempt to found a Jewish state will fail." — Adolf Hitler To Himmer and Heydrich. October 25, 1941
US to send citizens exposed to Ebola to quarantine camp in Kenya, not bringing patients home
