The words 'Granny's coming to live with us' has ignited many a row and enduring domestic feud in families.

By the time she was unpacking her housecoat, girdles, corn plasters and hairnet (whatever happened to hairnets? Every granny wore a hairnet at night after unhooking herself from her stout girdle) in the spruced up spare room everyone had calmed down, on the surface at least.

And there Granny – or Grandad – would stay, false teeth mug by the bed, and part of the family of their middle-aged children and grown-up grandchildren, great grandchildren, even, until their last breath.

Multi-generational families under one roof were simply how families worked – or had to work.

Fast forward a few decades, and granny's life was very different. Often separated from her grown-up children by hundreds of miles, living alone or, increasingly, the longer she lived, in sheltered housing or care homes.

Divorce and dissipated families meant, instead of all being under one roof, they had one roof each.

The need for carers rose, as the elderly grew older, more lonely and needed help, often staying in their homes, the mortgage long paid off and now worth a small fortune as house prices soared.

The cost of a small terraced house swelled way beyond the reach of their grandchildren, who were also priced out of the exorbitant private rental market.

So they went back home, going to bed every night, aged 29, in their childhood single bed, Enid Blyton books still on the bookshelves, wondering how their dreams had come to this.

The constantly shifting shape of family life through the decades is on the change again.

The boomerang trend of twentysomethings running home to mum and dad, who gave up all hope of early retirement long ago when any sniff of a pension receded to some day way into their seventies, even if then, has taken a new twist.

In our rapidly growing elderly population, 'children' in their seventies and eighties are going to live with their parents.

The number of over-70s in the UK has risen by 15pc in the past decade to just under eight million.

The number of people aged over 100 has increased by nearly three-quarters in the same period to about 14,500.

Elderly 'children' are now selling up and moving in, or relocating to the same sheltered housing or retirement complexes and villages where their parents, now pushing 100 or even older are enjoying old age.

The new dynamic of the 21st-century family – multi-generational families in retirement, with either the elderly children helping to look after their parents, or both sharing the care on offer.

As living beyond 100 becomes more expectation than great achievement, we are even looking at three generation living in old age together in retirement villages and sheltered housing, already falling far short of demand because the market just can't keep up.

Last year there were an estimated 295,000 multi-generational households, a 50pc increase from 10 years ago.

With dramatic improvements in life expectancy, families are coming together once again to keep down the costs of nursing or residential care.

Housing and care charity Anchor has noticed this trend with sets of parents moving into the group's villages with their own elderly children.

It's a trend that, all those years ago, when Granny, then only probably in her early 60s, was never predicted – Granny aged 105 living in communities of other elderly people with daughter aged 80 and grand-daughter in her early 60s.

As someone joked, communities where no one ever remembers to put the bins out.

But the reality of the new snapshot of family life is the tremendous burden on the health and care services by people who grew up believing that help would remain free for life.

It is far from the cosy picture it conjures up, and is cause for real concern.