The city, less than six decades old, had few grandparents. If they went out for the night and needed a babysitter for their four children, Mrs Ingwersen and her husband Frank found help from a co-op of young couples.

"When we arrived here, very few people had been born in Canberra," she said.

"Nearly everyone was from somewhere else. Most people were youngish, young families."

Now 76 years old, Mrs Ingwersen is part of a fast-growing age group changing the face of Australia's capital.

The ACT has had one of the steepest rises in its population aged 65 and older over the last two decades, new statistics show.

Australian Bureau of Statistics data shows 12.7 per cent of Canberrans were part of that group in 2018, up from 7.8 per cent in 1998. Only Tasmania had faster growth.

Mrs Ingwersen, of Higgins, is today one of the grandparents who were in short supply for ACT couples during the late 1960s.

Canberra then was a large country town of about 130,000 people, where parents relied more on neighbours and friends to help raise a family. The babysitting clubs helped couples meet others new to the national capital.

"It was a very young city back then," Mrs Ingwersen said.

The ACT remains one of the youngest places in Australia, having a median age of 35. Only the Northern Territory has a younger population among the states and territories. However demographers say Canberra could have more grey hairs in years to come.

The Council on the Ageing ACT says it issued 10,860 ACT seniors cards in 2017-18 - that's 19 per cent more than in the previous year. Community groups popular among retirees have also reported growing membership in Canberra.

The growth shouldn't take the ACT government by surprise. In 2009, it projected the population of people aged 65 years and older would increase by 214 per cent between 2007 and 2056. The proportion of senior Canberrans would rise to 20 per cent, it said.

Australian National University demographer Liz Allen said the transformation showed the city was, for many, no longer just a career stop.

"Federal government operations in the region have effectively made the ACT a quasi service population and, accordingly, people moved to work and then left to have families, further their careers or upon retirement," she said.

"What we’re seeing now is that more and more people are establishing themselves, partnering and building families and staying on in Canberra. This means people no longer see the ACT solely as a place to work but rather a place to live. And so the ACT population is ageing."

ACT residents typically have fewer children and longer lifespans than people in other states and territories, adding to its ageing population, Dr Allen said.

University of Melbourne demographer Peter McDonald said the ACT had a double baby boom in the 1960s and 1970s as its status as Australia's administrative centre grew and federal departments moved from Melbourne. Public servants followed them, and had children.

As baby boomers begin to retire, it remains to be seen whether they will stay or follow their kids interstate. Two of Mrs Ingwersen's children live in Canberra, encouraging her to stay in the city. A thriving scene of sporting clubs also makes it easier.

Her fellow Orienteering ACT members similarly moved to the city decades ago for work, or their partner's careers.

Bob Mouatt, who first arrived in 1962 to work in the air force, said it was easy for older people to stay active in Canberra. After making a living for decades as a carpenter, Eino Meuronen has adult children in the city able to look after him when needed.

Bureau of Statistics figures in 2016-17 showed more people aged 55-69 were leaving the ACT than arriving. The opposite is true among those older than 75 years old. Professor McDonald said they could be drawn to the territory by health facilities and nursing homes.

Canberra's changing age profile will require territory government attention, Council on the Ageing ACT chief executive Jenny Mobbs said.

"It means the government has to keep in front of the issue," she said, identifying suitable housing for older people as a challenge. The Property Council has observed a "missing middle" of medium-sized housing for couples wanting to downsize.

The ACT government has said respondents to a survey wanted more affordable, ground level, single-storey, and smaller properties. There was a need for more housing close to public transport, respondents said.

Ms Mobbs said the government would need to remember retirees' transport needs when planning the city. A national shortage of 100,000 home care packages would also need filling by the federal government, she said.

Professor McDonald pointed to opportunities for businesses in Canberra's ageing population. Many retirees would have time on their hands, and would be looking for entertainment.

Mrs Ingwersen, speaking after a hot afternoon's walk up part of Mount Ainslie with her orienteering club friends, said Canberra had lost some of its sense of community since the 1960s. However, its social life for seniors was one of the things she was enjoying most about the city.

Attitudes to ageing have changed too. In past decades, elderly people may have scoffed at exercising so much.

"It's not seen as being something strange," she said.

"You've got company, you've got people sharing your interests."