The proportion of people over 65 is rising in Balmain, parts of the north shore and on the northern beaches as more retirees "age in place" or downsize and low levels of new housing locks out younger families.
In the five years to June 2018 the median age across Sydney fell slightly from 36 to 35.8 years, but in many parts of northern Sydney, median ages rose by more than a year.
Balmain and the areas of Cremorne and Cammeray recorded a 2.5 per cent increase in people aged over 65 between 2013 and 2018 at the same time as residents aged 15-64 dropped by 3.5 per cent, Australian Bureau of Statistics data shows.
The Baby Boomer cohort - about 5.5 million people born between 1946 and 1965 - are shifting from work into retirement and while some choose a "tree change" or "sea change" away from the city, many more are staying in their suburban homes or downsizing to an apartment in an affluent area.
Crows Nest-Waverton, Neutral Bay-Kirribilli, St Leonards-Naremburn, Newport-Bilgola, Avalon-Palm Beach and Bayview-Elanora Heights all recorded rises in over 65s while people aged 15-64 made up less of the population mix over the five years to June 2018.
Sydney University planning expert Lawrence Troy said the relative lack of new housing in many of these areas has kept younger families out.
"There’s an old-world clashing with a new world," Dr Troy said. "In areas where there's much less new housing, there's much less opportunity for new households to move in.
"People are encouraged to age in place for a lot of reasons and that stops people moving but that's generally a good thing. There's a host of social connection and other more fuzzy reasons why people want to stay in the house they've lived in for years."
Tom and Inge Ferenci decided to downsize after 35 years of living in Riverview.Credit: Peter Rae
Macquarie University demographer Nick Parr said the rise of older people on the north shore and northern beaches is mainly due to Baby Boomers "ageing in place".
"These are all relatively prosperous areas which have longer life expectancies," Professor Parr said.
"But the areas where people are either just past or approaching the retirement age and form higher percentages of the population are mostly outer suburbs or beyond Sydney's urban fringe."
The Central Coast, Blue Mountains, northern beaches, Castle Hill, and parts of the Sutherland Shire are home to the largest Baby Boomer cohorts, he said.
"The location of many Baby Boomers in outer areas is a long-standing pattern which goes back to the previous century's pattern of young families tending to settle in outer areas," Professor Parr said.
The areas with the lowest median ages are close to universities. Kensington (25.8 years) and neighbouring Kingsford (26.5 years) are both located near the University of NSW.
Senior Research Fellow at UNSW Rafal Chomik said older people moving to regional areas as ‘sea-’ or ‘tree-changers’ is balanced by younger people moving into the cities.
"The contrast of flows of young people from regions to capital cities and opposing flows of retirees, mostly up and down the eastern seaboard, to places like the Sunshine Coast in Queensland, Central Coast in NSW, and Mornington Peninsula in Victoria, is likely to accelerate as we grow older as a nation," Mr Chomik said.
In the five years to June 2018, the median age increased from 41.5 to 42.1 years across regional NSW. The areas with the highest median ages in 2018 were on or near the coast.
Tea Gardens-Hawks Nest, near Port Stephens was the oldest - 62.7 years, Tuncurry (61.1 years) on the Mid North Coast, and Sussex Inlet-Berrara (59.2) on the South Coast are all popular retirement destinations.