It’s a hot day in rural Mississippi, and a family of four has been on the road for hours. Everyone’s tired and a little bit sweaty, and the kids in the back are getting bored. Soon they’re shoving each other in the back seat, name-calling, bickering and pushing those oh-so-familiar sibling buttons. The father takes a deep breath, swerves the car off the road and pulls to a halt outside a scruffy little blues bar – but he’s not shouting at the kids. Instead, everyone piles out and they go in: mum, dad and darling daughters. Frosty beers are downed all round and holiday happiness is restored. The end.

It’s not your typical family holiday narrative. But then, this isn’t your typical family holiday. All the classic ingredients are there, but there’s one crucial difference: the kids are in their twenties. And one of them is me.

So what would possess a young adult to waste a precious week of annual leave on a holiday with their stuffy old parents, you may be asking yourself. Or, if you’re older, you might wonder why any self-respecting empty-nester would waste their time and money travelling with a pair of overgrown teenagers. And yet the latest trend in travel – grown-up family holidays – is fuelled by people who see no problem in holidaying across the generational divide.

Holidaying with parents is no longer for losers. My sister is a model of successful twentysomething-dom. She’s an architect who likes to wear structural clothing from Cos, read existential philosophy and eat elaborate brunches around East London. And yet, like me, last year she was one of those two kids shoving each other in the sticky back seat while the Mississippi swamp rolled by. Her sophisticated colleagues might be surprised to discover that she can spend hours fighting to push me off an inflatable unicorn into a swimming pool, or jabbing me in the ribs before cackling and running away. Family holidays, inevitably, go hand in hand with a certain level of regression. You could say it’s part of the fun.

My family is far from the only one still going on trips long after the children have moved out. A rough survey of my friends – all now several years post-university – reveals that around half of them are doing the same thing. And yet this does seem to be a new phenomenon. A friend tells me that her mother’s older colleagues are baffled by the idea of grown children going on holiday with their parents. So what’s changed?

“What would possess a young adult to go on holiday with their stuffy old parents?”

There are several factors at work. First – of course – there’s the question of money. Whereas at our age our parents might have been living in their own flats, perhaps engaged, and with their first major job, we’re juggling “portfolio careers”, living in large, student-like house shares, and writing passive-aggressive Post-it notes about Sainsbury’s Basics lentils. Some of us are living back at home. All of this leads to a certain level of extended kidulthood – no wonder it extends to holidays too. Think of it as the travel agent branch of the bank of mum and dad.

And then there’s the quality of the trips on offer. A friend of mine tells the story of a recent, nightmarish holiday with friends from law school. “We were all on a budget, so we got a villa that was cheap because it was in the middle of nowhere. There were eight of us, but we only rented one car – which was a terrible decision.”

The experience was enough to make her nostalgic for the days of luxey family villas and free meals. Not that millennials are solely driven by mercenary matters. It turns out a lot of us actually like our parents and enjoy spending time with them. As my lawyer friend says: “I now prefer to go on major holidays with family. We have similar agendas. We’re all very outdoorsy. Often our goal is to see a specific animal or bird.” Among friends, her love of nature is cause for hilarity; with her family, she can just enjoy it.

It’s hardly surprising that our holidaying styles are shaped by our parents and by our early holiday experiences. Someone used to adventurous, seat-of-the-parachute-pants travel since childhood is unlikely to be happy on a package tour to Kavos with their uni friends. The people we have the most in common with are the ones who shaped our tastes in the first place.

As for the parents, they think it’s marvellous: you can have experiences together that would have been impossible with younger children in tow, all rounded off by a chilled glass of Picpoul or a Mojito. This year my family went on a ten-hour train ride through Sri Lanka. We were in the “Observation Saloon”, a delightfully old-fashioned carriage with seats that faced backwards, towards the huge window that formed the rear of the train. We spent the day watching the Sri Lankan countryside recede with dumbstruck smiles on our faces.

So what are my tips for a successful adult family holiday? Firstly, don’t make it too long: we reckon 10 days is our limit. Secondly, get everyone involved with the itinerary. Nothing transforms a young adult back into a sulky teen faster than being dragged around cultural sites that don’t interest them. Thirdly, remember your headphones. There will be times, particularly on long drives, when checking out of reality is, paradoxically, the only way to keep your sanity.

And lastly, don’t try too hard to fight the regression. For the harassed millennial still reeling from the demands of adulthood, a bit of childishness can be extremely welcome. For parents edging towards retirement, some silliness can be pretty welcome too. When we’re on holiday as a family, we don’t have to be serious, or impressive, or even competent. Just like when we were young, my sister and I tease each other mercilessly. My parents join in. The difference is, now that we’re grown-ups, it makes us laugh instead of cry.

Think of it as the travel agent branch of the bank of mum and dad