The relation between the number of people who aren´t of working age and the number of people who are is captured in the dependency ratio.
In Ireland during the sixties, when contraception was illegal, there were ten people who were too old or too young to work for every fourteen people in a position to earn a paycheck.
That meant that the country was spending a large percentage of its resources on caring for the young and the old.
Then Ireland´s dependency ratio hit an all-time low: for every ten dependents, it had twenty-two people of working age. That change coincided precisely with the country´s extraordinary economic surge.
Demographers estimate that declines in dependency ratios are responsible for about a third of the East Asian economic miracle of the postwar era; this is a part of the world that, in the course of twenty-five years, saw its dependency ratio decline thirty-five per cent.
Dependency ratios may also help answer the much-debated question of whether India or China has a brighter economic future. Right now, China is in the midst the "sweet spot".
In the nineteen-sixties, China brought down its birth rate dramatically; those children are now grown up and in the workforce, and there is no similarly sized class of dependents behind them. India, on the other hand, reduced its birth rate much more slowly and has yet to hit the sweet spot. Its best years are ahead.