When the 50-year-old Madonna regularly steps out in the clothes of her 12-year-old daughter, you know a moment has arrived. The fashion industry is calling it “transgenerational dressing”, but the truth is mums and daughter have been pilfering each other’s clothes for a while now. Afraid of the whole mutton thing? Don’t be. While you may have long been eyeing up your mum’s wardrobe for vintage factor, did you ever consider the fact that your offspring could be your most useful new styling tool? Be warned, though — you have to be prepared to let it work both ways. You can’t be precious about her eyeing up your prized Vuitton leopardprint scarf, when her American Apparel leggings will be perfect under your tunic dress.

“The idea of me raiding my mother’s wardrobe when I was a kid is ridiculous,” says the retail guru Mary Portas, “but now I swap clothes with my 13-year-old all the time. Lots of rules about what you can and can’t wear at a certain age have gone out of the window, and lots of our high-street brands surf the generations really well.” Cheap Monday skinnies, Gap shirts, Converse All Stars, Topshop knits — all are fair game in the Portas household, where “my housekeeper is always putting Verity’s things in my wardrobe, and vice versa”.

The concept has been successfully exploited by the French brand Comptoir des Cotonniers, whose ad campaigns feature real-life mothers and daughters wearing the clothes. Each season the brand receives more than 10,000 entries from mums and daughters keen to feature in the campaign — only six end up being chosen.

Kathy Hill and her 17-year-old daughter, Tess Jantschek, are the only British couple to have featured in a Comptoir campaign. Hill says her daughter is an invaluable resource on their joint shopping trips. “I value her opinion,” she says. “Tess being young and trendy rubs off on me. She updates my look for me, and I find myself looking at the things she would. On the flip side, being a teenager, she can have a tendency to go over the top. I can rein her in a bit.” They often share clothes. “It helps that we’re both a dress-size 10, but, annoyingly, Tess’s feet are smaller than mine, so I can’t borrow her shoes.”

It was the teentastic Topshop that really introduced the idea of TG dressing. “You can shop there until whatever age you want,” says the stylist Bay Garnett, although Portas says: “I wouldn’t buy my dresses or more directional pieces there.”

For while the mutton/lamb lines have been blurred, there are still rules to observe if you’re going to get TG dressing right. In a recent interview, Georgia May Jagger, 17, described cleansing her mother Jerry Hall’s wardrobe of miniskirts. “I didn’t say she couldn’t wear them, but I told her one night that her skirt was too short — she came down the stairs and I was, like, ‘God, Mum, you are 50.’ ”

Minis are also out for Portas, “unless heavily supported by thick Wolford tights”, while sleeves “become vitally more important at a certain age, unless you’re a sinewy little thing like Madonna”.

Your daughter can be a brilliant gauge as to how far, or not, to go with high-fashion trends. “When I see certain things on Verity’s cute little teenage body, it makes me realise I can’t wear them,” Portas says, also citing a pair of gold McQueen for Puma trainers she received as a gift that went straight into Verity’s closet. “I took one look at them, and was, like, ‘Get outta here.’ ”

The same lament is voiced by Pearl Lowe, whose daughter Daisy first started wearing her clothes aged 16: “She’s a 39 and I’m a 40, so she can wear my shoes, but hers hurt me.” Daisy remedied the situation by buying Lowe a pair of patent Westwood lace-ups, followed by a pair of Louboutins for Christmas. “I stopped wearing heels when I moved to the country, which Daisy thought was really boring,” Lowe says.

Or perhaps her daughter was trying to make up for the time she took a pair of Vivienne Westwood pirate boots without her mother knowing. “I remember calling her in New York, as I wanted to wear them that night, hoping she might know where they were. She guiltily said she would return them asap, but that wasn’t any use to me,” Lowe says. Thievery, naturally, is going to be an issue in any TG-dressing household. We say: threaten your daughter with a grounding, and keep your Lanvin under lock and key.

Another potential pitfall is the fact that, as Lowe puts it: “She looks better in everything that I have, especially as I am getting older and she is looking more and more beautiful as she enters her twenties.” Hill is less bothered: “Tess wears a lot of my old 1980s D&G and Azzedine Alaïa, and she looks lovely in it. I never feel jealous: it gives me a buzz seeing her in them.”

Fashion-forward mums archive key pieces for their baby daughters: Tamara Mellon even uses Elika Gibbs from the company Practical Princess to create special storage boxes for the things she will keep for her daughter, Minty. Sadly, the Ossie Clark dresses Lowe had put away for Daisy were “sold one day when I needed the money” — although it also worryingly suggests future generations of teenage girls teetering into school on 4in Jimmy Choos, clutching your 20-seasons-old It bag.

Back to this season, and items to pilfer from her wardrobe that will instantly update yours include bright jersey basics (for colour blocking), boxy blazers, jeans with rips and any headpieces or flamboyant jewellery she brings home from Topshop’s Freedom counter. And a final word of advice: the 1980s really won’t work on you the second time around. So, hands off her acid-wash denim jumpsuit — you have been warned.

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