A an ever larger proportion of a population is too old to do significant amounts of work, the smaller proportion of healthy able bodied younger people is forced to pick up the slack. This goes from teachers, to truckers, to plumbers and beyond.

This burden is made even worse as most developed countries have established pension systems that act as generational wealth transfers, where younger workers pay for the pensions of the elderly, in the hopes that the next generation of children will in turn pay for their parents' pensions.

It gets worse: that large mass of retired people now represent the main block of voters, making it very difficult to enact pension reforms that will reduce the amount of money they siphon from younger generations.

It is very easy to go from a sustainable stable population to a rapid population decline that is difficult to recover from.