Falling fertility rates could mean most countries see their populations shrink by the end of the century.
The world will have to begin to reckon with the consequences of a smaller, older population.
1. It could be good news for poorer countries
We're looking at very different situations in different parts of the world.
Falling fertility rates - the number of live births per woman, according to the official definition - and economic development tend to go hand in hand.
Better education and career opportunities for women, access to contraception and abortion and lower child mortality rates, mean women on average have fewer children.
So for lower-income countries, a falling birth-rate could spell better living standards.
A smaller number of children each get a bigger piece of the pie, whether that's health or education.
But in countries where fertility rates have already been falling for years, shrinking further could cause problems.
These countries will have to work out how to care for a growing older population, with fewer younger people to work as carers and to pay into the system.
2. People might have more to look forward to in retirement
But they might have to work for a lot longer.
And they might not be as much of a strain on the healthcare system as feared.
A lot of the worries about caring for an ageing population assume everyone will be ill in old age.
But as well as life expectancy, the world has been making gains when it comes to "healthy life expectancy".
In pretty much every country around the world, with the notable exception of Syria, new babies are expected to spend more years in good health than those born in the year 2000 - five extra healthy years on average.
In Rwanda, the average baby has gained 22 additional years of expected life in good health since the start of the millennium.
In higher-income countries like the UK, Germany and the US, healthy life expectancy has increased by between one and three years.
"The fears around an ageing population have to be put into perspective," says Prof Sarah Harper at the Oxford Institute of Population Ageing.
"The health of older adults is already much better than it was," even a few decades ago, she points out, meaning older people can be "active, healthy" and paying in for a greater proportion of their lives.
And, as Dr Hannah Ritchie at the University of Oxford's Our World in Data team points out: "We don't even know what the world of work will look like in 50 years".
3. Governments might have to open borders
Fertility rates and life expectancy are two parts of the equation when it comes to whether a population is growing or shrinking. The third is migration.
Countries that end up with much smaller populations of young people might want or need to attract young people from elsewhere.
The world could become even more culturally and ethnically mixed, says Dr Ritchie.
4. It will pay to support parents
When governments have tried to restrict or increase a country's birth-rate in the past, it's often been coercive.
But there are examples - notably in Scandinavian countries - where birth-rates are higher than expected because of incentives like generous maternity leave and childcare.
So in the future, wealthy countries choosing to introduce generous support systems may see an uptick in their fertility rate, according to Dr Ritchie.
As much as women in lower-income countries may have more children than they'd ideally choose, some women - and men - in countries with high costs of living might have fewer children than they'd like because they can't afford more.
Alongside this, governments may well increase the pension age - possibly even allowing people to take a chunk of time off to raise a family, and then work that extra time later in life, Prof Harper suggests.
5. Carers will be "as important as doctors"
That's the take of Dr Tiziana Leone at the London School of Economics.
No matter how big the gains in healthy life expectancy, the "oldest old" will probably always need care towards the end of their lives.
Dr Leone warns countries with ageing populations face a crisis in terms of their health and social care systems.
We need to start now, by training the right workforce - "we'll need fewer paediatricians and gynaecologists", she says.
6. It will probably be good for the environment
A shrinking population is "a good thing" for the environment, according to Prof Harper.
But Dr Ritchie points out that economic growth is a stronger driver of climate change than population growth.
It's extremely difficult to say what will happen to the state of the economy over the longer term.
If the world becomes richer and consumes more despite the numbers of people shrinking, environmental gains aren't guaranteed.
Equally, while wealth and pollution have been linked over the past century, in recent years it's the richer countries that have been able to reduce their CO2 emissions by investing in technology.
And this pattern could continue.
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