"All social planning is based on population size but also on age structure, and that fundamentally changes in a way that we still do not understand," said George Leeson, CEO of the Oxford Institute of Aging Population to the BBC.
The Institute of Measurement and Health Assessment (IHME) at the University of Washington, Seattle, was published in The Lancet and compares public health in the world between 1950 and 2017.
In almost half the world In developing countries, especially in Europe and North and South America, not enough children are born to maintain the size of the population. Something that will have great consequences when communities get more "grandparents than grandchildren".
The result came as a "big surprise" for the researchers, the BBC writes.
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Since 1950, childbirth in the world has fallen by almost half, from an average of 4.7 children per woman to 2.4 children per woman in 2017. But the variations are great, write the researchers. In Africa and Asia, childbirth continues to increase with the average number of women in Niger who feed seven children during their lifetime.
According to the IHME, Cyprus is the least fertile country in the world – an average Cypriot woman is giving birth to a child in her life. On the other hand, women in Mali, Chad and Afghanistan have an average of more than six children.
Ali Mokdad, a professor at IHME, says that education is the most important factor for population growth.
"If a woman trains alone, she spends more years at school, postpones pregnancy and therefore has fewer children," he says.
Mokdad says that while populations in developing countries continue to increase as economies in general are increasing, which generally has a decreasing effect on childbirth over time.
"Countries are expected to improve economically and fertility is more likely to decrease and stabilize.
The critical point is when the average fertility level of a country reaches 2.1 children per woman. Then childbirth begins to decline. When the study began in 1950, no country has come to that.
"We have reached a basin where half of the countries have fertility levels below the level of compensation, so if nothing happens the population will decline in those countries." This is a remarkable transition, says Professor Christopher Murray at IHME.
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The fact that the birth rate is falling in many rich countries does not mean that the population also does, because the population of a country is a mixture of childbirth, death and immigration. It may also take a generation before the shift begins to notice, but as more countries get better economies, the phenomenon will become more common, according to researchers.
We also live longer than ever. The overall expected life expectancy for men increased to 71 years, from 48 in 1950. Women are expected to live up to 76 years, compared with 53 in 1950.
Heart disease is nowadays the most common cause of death in the world, says the IHME. By the end of 1990, there were neonatal problems, followed by lung disease and diarrhea.
"You see less mortality from infectious diseases as countries get richer but also more disabled because people live longer," says Ali Mokdad.
He noted that although deaths from infectious diseases, such as malaria and tuberculosis, have declined significantly since 1990, new noncommunicable diseases have occurred.
There are certain behaviors that lead to more cases of cardiovascular disease and cancer. Obesity is number one – it increases every year and our behavior contributes to it, he says.
If development does not break, we will have a population development with few children, but with many ages.
To counter the consequences of a declining population, there are three things a country can do, write the researchers: Increase immigration, get women to feed more children with policy reforms, and raise the retirement age.
None of the measures was successful, however, the study says.
Countries with generous immigration are struggling with social and political challenges, the blockade to increase birth rates did not have a major impact on fertile women and proposals for the older retirement age were often met by protests.
Migration for young people from poor countries move to rich countries, nor is it a solution at the global level, according to the study.
George Leeson is still optimistic and believes that an aging population need not be a problem as long as it is adapted to society.
Demography affects all parts of our lives; traffic, as we live, consumption. It's all about demographics, but we have to plan an altered age structure in a way that we still do not understand, he told the BBC.
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