"All social planning is based on the size of the population, but also on the age structure, and it is fundamentally changing in a way we have not yet understood," said George Leeson, executive director of the Oxford Institute of BBC aging.
The Institute for Measurement and Evaluation of Health (IHME) at the University of Washington in Seattle was published in The Lancet and compares public health in the world between 1950 and 2017.
In almost half of the world the countries, mostly in Europe and North and South America, do not work enough children to maintain their population size. Something that will have big consequences when communities get more "grandparents and grandparents than grandchildren".
The result was a "big surprise" for researchers, BBC writes.
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Since 1950, worldwide delivery is almost halved: from an average of 4.7 children per woman to 2.4 children per woman in 2017. However, variations are large, writes scientists. In Africa and Asia, childbirth continues to grow with the average Niger women who feed seven children throughout their lives.
According to IHME, Cyprus is the most disadvantageous country in the world – an average Cypriot woman raises a child in her life. On the other hand, women in Mali, Chad and Afghanistan have more than six children on average.
But Mokdad, a professor at IHME, says the most important factor of population growth is education.
"If a woman trains, carries out many years at school, postpones her pregnancy and hence has fewer children," he says.
Mokdad says that while populations in developing countries continue to grow, so their economies are generally increasing, which usually reduces the impact of birth over time.
"It is expected that countries will become better economically, and it is more likely that fertility will be reduced and equalized.
The critical point is when the average fertility level of the country reaches 2.1 children per woman. Then the birth begins to decrease. When the study started in 1950, no country had reached that point.
"We have reached water where half of the countries have fertility levels below the level of compensation, so if nothing happens, the population will shrink in those countries. This is a remarkable transition, says Professor Christopher Murray at IHME.
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The fact that the rate of birth falls in many rich countries does not mean that the population also does because the nation's population is a mixture of birth, death and immigration. It can also take years before the change starts to notice, but as more and more countries get a better economy, that phenomenon will become more common, according to researchers.
We also live longer than ever before. The expected global lifespan of men has increased to 71 years since the age of 48 in 1950. Women are expected to live at 76, compared with 53 in 1950.
Today is a heart disease the most common cause of death globally, says IHME. Until 1990, there were neonatal problems, followed by pulmonary disease and diarrhea.
"You see less deaths from contagious diseases, because the countries become richer, but also more disabilities, because people live longer," says Ali Mokdad.
He noted that although mortality from infectious diseases such as malaria and tuberculosis has been significantly reduced since 1990, new non-transmissible diseases have occurred.
– There are certain behaviors that lead to more cases of cardiovascular disease and cancer. Obesity is number one – it grows every year, and our behavior contributes to that, he says.
If development does not cease, we will have the development of population with few children, but very many age groups.
To counter the consequences of a declining population, the country can do three things, and researchers say: Increasing immigration, encouraging women to have more children's political reforms and raising retirement age.
None of the measures was successful, however, says the study.
Countries with generous immigration are struggling with social and political challenges, locking to increase the birth rate has had no major impact on fertile women, and proposals for senior retirement age have often been met by protests.
Migration, From Youth From poor countries move to rich countries, and this is not a global solution, the study says.
George Leeson is still optimistic and believes that aging of the population does not have to be a problem, provided it is tailored to society.
Demography affects all parts of our lives; traffic, how we live, consumption. This is all about demographics, but we have to plan the changes of the age structure in a way we have not yet understood, he told the BBC.
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