“But pay board – it’s great practice managing money and you might even be able to convince your mum and dad, if they can afford it, to stash your cash and return it to you later,” I disloyally divulge.

Because, for parents, the “afford it” bit is the rub. And you can’t have missed last week the rather entertaining story of a US couple who turned to the courts to evict their 30-year old s(p)on(ge).

Well, the collective parental cost of Australia’s stay-at-home adults is now $12.2 billion a year, according to a mozo.com.au survey of 1000 Australians, given exclusively to Fairfax Media. That frighteningly fat figure is calculated from ABS data and the spend reported by parents in this survey.

Almost one in three adults under 35 remain at home, analysis of ABS data indicates. (It’s on the increase too: the 2016 census showed a 20 per cent jump in the number of 25 to 34-year-olds sticking around … to nearly 400,000 across Australia).

But 60 per cent of these stay-at-home adults between the ages of 19 and 34 pay no rent, Mozo found. And 75 per cent don’t contribute to the household bills either.

Instead, one-in-three parents are forking out between $101 and $200 a week for their so-called kidults, including accommodation, internet, transport, power and other expenses. Another one-third pay $51 to $100.

And that seems like a bargain to reader Lianne Lewis.

“I reckon they cost us about $20,000 a year – each,” says Lianne, whose two sons, Ethan, 22, and Declan, 18, still live with her and her husband Mick Carlsson at their home half an hour out of Sydney. “I think they probably go through at least $150 a week in food alone.”

Does Lianne begrudge the large (but the survey says not unusual) amount? “No – they’re my sons and I love them. We want them to optimise their life and have a great start – I think that’s the deal you take on when you have a child.”

Nearly nine out of 10 parents of stay-at-home adults are similarly happy to help.

Which is lucky because almost three out four of their children are there because they can’t afford to leave, and parents from across the country tell me low youth pay rates along with inconsistent hours are big factors.

Says Rockhampton-based spare-parts-manager Lloyd Swadling, father to stay-at-home son Kurt, 21: “He works part-time for Bunnings while he pursues his acting career. Needless to say money is tight as his shifts drift up and down.”

Meanwhile, one of Kurt’s younger brothers, Dane, 19, has just returned home after six months away – “he couldn’t afford rent on an apprentice wage” – in what’s become known as a Boomerang move: many children attempt to make it on their own and then come back.

Lewis puts it well: “It’s just a little bit expensive to be a young person now … let’s talk about casualisation and exploitation and wage theft and house prices – I mean it’s just a perfect storm.”

But when I ask about her and Mick’s own future, Lianne concedes: “It’s going to have a big impact on our retirement.

“If they were not here that would free up $40,000 every year to put into shares or super or a house or whatever.”

Owners of an electrical contracting business, they are “very aware” of what they’re missing in compounding but have modelled their super and “should be OK”.

It helps the couple is only letting their boys stay without board while studying; the rule is they need to pay once they’re paid. That means Ethan, who’s in third year law at Macquarie University, is currently living rent-free while Declan, who has decided to wait and work a year before going to university, hands over a “token” $50 a week.

It’s the same deal Maree Ram, a primary teacher on Queensland’s Sunshine Coast, made with 21-year-old daughter Alyssa: “I said ‘If you’re not studying, you need to pay board’ – it’s earn or learn.

“We don’t give her any money and she buys all her own clothes and everything. She can eat what’s in the house but we won’t buy anything special, the basics.”

Indeed, no parents I spoke with intend giving a ''free lunch'' for life.

Maree adds that Alyssa, who’s now studying psychology, initially moved away aged 18. It was the “best thing we ever did as a family” because it made her realise what it cost and took to run a house.

When she returned home, like thousands of Australia’s cash-strapped kidults, Alyssa began to contribute in other ways. In what represents a big saving for Maree and her husband, Prem: “She babysits [younger sisters, Stella, 10, and Amelie, 11] for free without any complaints. She’s pretty responsible.”

One in 10 adult children at home do the same, says the survey, while nearly 60 per cent compensate their parents by cooking, cleaning, and doing laundry and other household tasks.

For Ethan, Declan, Kurt, Dane, Alyssa and all stay-at-home adults, I have a vital message: Your parents are compromising their own lifestyle – now, later or both – to fund your life.

"Repay" them by seizing the once-in-a-lifetime chance to set yourself up … for life.

Nicole Pedersen-McKinnon is a financial educator who delivers Smart Money Start in high schools around Australia.

Sydney Morning Herald