By this definition, a third of Australians – about 7 million people – are now middle-aged, say the Edgars in a new book.

As for when someone is officially old, they argue that the new old is 75, although many people will still have 25 years of life ahead.

In Peak: Reinventing Middle Age, they call for a radical and creative rethink of perceptions, portrayals and policies affecting the new middle age.

Instead of being old at 50, they say we are at our peaks, our prime, our most productive, bringing together wisdom, experience and social intelligence.

"By 50, we are at the midpoint of our productive adult life. This is our peak; these years are the gift medical science has given us," says the book.

Yet the over 50s are often derided as useless, say the Edgars. They are often depicted in the media as blind, bumbling and bent over like Mr Magoo in 1950 cartoons, Their contribution is undervalued, yet their work as unpaid carers and volunteers is the glue that binds society, said Dr Edgar.

"They're diminished, yet society couldn't function if they didn't do (these roles)," he said.

Mrs Edgar, now 79,  was the first woman appointed to the Australian Broadcasting Control Board in 1975 and founding director of the Australian Children's Television. Her husband, Dr Don Edgar, now 80, founded the Australian Institute of Family Studies.

A century ago, most people lived 25 years less than they do now, and middle age was considered to start in the 40s.

By 2060, about 25 per cent of the population will be over 65 years, they write.

In 2012, there was one person aged 100 or more for every 100 babies. By 2060, there will be 2.5 centenarians for every 100 babies.

The Edgars argue that policies and attitudes are still bogged down in the perception that 50 is the beginning of being old: Ads target the over 55s for retirement villages, 50 is often a benchmark for redundancy at work, and 50 is when people qualify to join the University of the Third Age.

"Our policy discussions fail to address the new realities of longevity," the book says.

Negative stereotypes had a physical impact, and we had to stop talking about middle age as if it is a terrible thing.

Dr Patricia Edgar said research had shown that when people who were ageing were denigrated and made to feel useless, their health suffered. They walked more slowly, they heard less.

Her husband, Dr Don Edgar said the failure of policy makers and businesses to recognise the value of this group was a huge waste of resources.

"They could be used in mentoring younger people, but the usual attitude is pushing them out the door," he said. Instead of offering reduced hours or a changed role, many over 50s were targeted for redundancy.

"We've got to talk about flexibility over a longer life span," he said.

The Edgars say they have never felt old.

"I had a lot of trouble with that, defining myself as old," said Dr Patricia Edgar about the definition of old as 75.

"We have never stopped planning what we will do next, and we have a very full life."