Does the former banker think her son James treats her home in Terrey Hills like a hotel? "Absolutely, " Ms Di Carlo said with no hesitation. Does she mind? "It doesn't bother me one bit."

Occasionally for "selfish reasons," Ms Di Carlo, who also has a 10-year-old son living at home, would like James to move out. But mostly, she loved having him at home.

Far from seeing their kids as babygloomers or parasite singles, as these young adults are called in Japan, Ms Di Carlo is typical of many parents who are enjoying the company of their adult children, finds a new study by Deakin University's Elyse Warner.

While some children felt there was a "stigma" attached to living with mum and dad, the boomerang kids returned home for financial reasons.

Nearly a quarter of Australians aged 20-34, more commonly men, still live with their parents. Of those aged 25-29 who live in the parental home, more than half have moved out and returned. A previous study found two-thirds of those who returned home were motivated by money.

While returning home to live is often perceived as "quite a negative life event", Deakin University's interviews with nine adult children found it was usually the opposite.

“The results of our study clearly challenge the notion that returning home is a backward step for young adults,” Ms Warner said. “The majority of the young adults felt that returning home was not regressing; it did not necessarily mean they had lost whatever they had gained while living independently. Like their friends and other young people, they were still realising their personal goals and plans for their futures, whether through work or study. For the most part, moving home was only short-term in the scheme of things."

Ms Di Carlo said most of "the kids" in Terry Hills lived at home like her son. "They can't afford to live away from home. Some pay board, but James doesn't."

Most of James' friends were obsessed with cars and motorbikes and showed no sign of leaving home.

"Because there's no girls involved, there's no inkling that they want to move out. Boys have more freedom now, and they don't need to move away from home," said Ms Di Carlo.

"I moved out at 20 to marry. My father was Italian and you didn't leave home until you married, and you didn't have a string of boyfriends. You found a boyfriend, got married and had a family."

According to the 2011 Census, around 21 per cent of Australian households comprise adult children over 18 years of age. The biggest growth in stay-at-home, or boomerang kids, occurred in sole parent households, where the number of adult children 18 or over jumped 17 per cent.

In Fairfield , 36.2 per cent of all families have adult children over 18 years of age living at home. Like other areas with large numbers of migrants, it has seen large increases in the number of children either staying or returning home. It is the same story in Bankstown, with 31.9 per cent,  Liverpool, with 30 per cent and Campbelltown, with 29.5 per cent.

It is a global trend, occurring in Japan, Europe and Scandinavia. In the USA, 29 per cent of 25- to 34-year olds have either never moved out of their parents' home or have returned home because of the economy, found research by the Pew Foundation. Around 53 per cent of 18- to 24-year Americans lived at home.

Is failure to launch a failure of parenting? Far from it, says a new body of research that suggests adult children living at home generates tolerance and understanding between generations.

American authors Richard Settersten, Ph.D., and Barbara E. Ray, argue a slower path to adulthood is good for everyone, allowing young adults to get a better start in life and on home ownership. They interviewed more than 500 people aged 18 to 34 for their book, Not Quite Adults: Why 20-Somethings Are Choosing a Slower Path to Adulthood, and Why It's Good for Everyone, to prove that these babygloomers aren't slackers.

They found staying at home was an economic necessity. Their research in the United States showed 43 per cent of young people between 25 and 34 would be in poverty if they weren't living at home; living at home makes the rate only nine per cent.

"We're also not saying that young people shouldn't assume responsibilities for themselves or others; they must. But we should be far more worried about young people who jump quickly into marriage or parenting before they're ready, who leave home without adequate resources, and who bypass higher education - these are all extremely risky ventures today."

On their blog, Settersten and Ray say these new living arrangements are creating uncertainty for parents who don't know what to do "when the guidebooks end at the age of eighteen. There are no norms for 'big' kids who … have yet to achieve the major milestones of adulthood. Relationships are being crafted as they go."

For example, how do adult parents deal with the division of chores or the question of whether boyfriends or girlfriends should be able to stay overnight?

So has Ms Di Carlo had "the talk" with James about girlfriends staying over?

When her son had a party, he asked if a former girlfriend could stay the night. "I said 'that's okay' because I didn't want anyone driving, but I told him not to make a habit of it."

Ms Di Carlo said adult children were also staying at home longer because values had changed.

"I am not as strict as my father was. We had everything material when I was growing up. My first car was bought for me, but I had no freedom. I couldn't do anything, I couldn't go out with a boy."

How to live happily (hopefully, not forever after) with your adult children

Nobody knows how many adult children living at home pay rent or contribute to bills.

Anecdotal evidence suggests even those that do make a financial contribution, by paying rent or contributing to the cost of food, rarely pay enough to cover the real costs, including the cost of wear and tear or utilities.

To avoid resentment, Mark McCrindle, a social analyst with McCrindle Research, says families should try to agree on a budget and a plan. Very few parents and boomerang children have a formal arrangement covering costs and the length of the "tenancy" or live in arrangement, says Mr McCrindle.

He says families should discuss:

Costs: As well as a discussing food costs, parents and adult children should also discuss the cost of utilities, which are often large but not included in discussions.

Contribution to domestic duties: If not organised ahead, these responsibilities often fall on the parents' shoulders, particular women who account for nine out of 10 carers in Asutralia.

Time: It is often hard to forecast how long the arrangement will last, but McCrindle says there should be an exit clause, at least a scheduled time to reassess the arrangement.

McCrindle says very few approach living arrangements like this. Without discussion, kids often fall back into old habits. "Kids have lived at home for two decades (before they moved out and returned) so it is hard not to fall back into old patterns and take advantage of mum and dad."