WASHINGTON - By 2100, babies born today will be 78 - and will live in a world in which older people outnumber the younger.

But from Europe to Japan, this demographic change is already apparent.

According to a study published in The Lancet in July 2020, while the number of people on the planet will peak at 9.7 billion in 2064, by 2100 as many as 23 countries are expected to see their populations halve, with the global population falling to 8.8 billion.

"We're really talking about what we call age structural change" said Dr Sarah Harper, professor of gerontology at the University of Oxford and director of the Oxford Institute of Population Ageing.

"And that is as much about falling child-bearing as it is about falling mortality rates, late-life mortality rates," she said on The Straits Times' latest Conversations on the Future.

"Probably most countries across the 21st century will age, and we will move from societies where we have lots and lots of young people being born and coming into our economies, and driving them, to societies where most of the population are mid-life or over.

"But that doesn't mean we're going to have a population of old, frail people. It just means that... instead of in a population pyramid, we're going to be living in more of a skyscraper," she said.

"And by the end of the 21st century, we'll have gone through that transition. But it's what's happening across the 21st century that I think people find concerning."

She added: "I think the really important thing is… (to) talk about economic dependency ratios, that is the number of workers that are coming into our societies and economies."

The key to adaptation is to stay healthy for longer.

"As we are living longer, it's really important that we keep healthy," Dr Harper said. "If we can keep our population healthy for as long as possible, then there's no reason why, as older adults, we shouldn't be contributing to our society."

Dr Harper has chaired the British government's Foresight Review on Ageing Populations, and has been on the country's Prime Minister's Council for Science and Technology.

It is really important for governments to take a holistic view and try to make societies and individuals resilient, she said, adding: "There are some key countries that I think are really beginning to prepare.

"If we look at what's happening in Japan and Korea, for example, and Singapore, these countries have very good forward planning, where they're really beginning to look at it from a holistic point of view. Australia is another example."

But she cautioned against "institutional (or) structural lag", which means still using "21st century institutions and ways of thinking to tackle this 21st century problem".

Asked if, with advances in fields such as household robotics and communication, the end of the century will be a lonelier world for older people, Dr Harper said: "Technology can be really helpful, but the interface between people and technology is the area we really have to focus on.

"Technology can connect, but we must be careful we don't move into a world where… we have families and caregivers saying, 'Well, robots can do that. And therefore we know mum is fine because we've got her a robot'."

She added: "And of course, it's about us. Sometimes we talk about older people of the future as though these people are just going to emerge. But they are us. It's basically almost everyone who is alive at the moment."

The Conversations on the Future series focuses not on current news but on broader, and larger, long-term issues and trends.

Among the interviewees are Harvard professor Graham Allison, historian Wang Gungwu, science fiction writer Chen Qiufan, Yale Law professor Amy Chua and diplomat Tommy Koh.