Twenty years ago, no teenager in their right mind would consider wearing the same clothes as their mum. Let alone their gran.
But last weekend I watched with fascination as three generations of women queued up for the changing rooms at TopShop, almost identically dressed in skinny jeans, gladiator sandals and slouchy knitwear. All three had swishy shoulder-length bobs, golden tans, perfect manicures and statement handbags. And they all appeared to be roughly the same dress size.
From behind, no one could have guessed that there was at least a forty-year age gap between the oldest and the youngest – and I'm pretty sure that my jaw dropped when I heard one of the 'girls' called the other ones 'Nan' and 'Mum'.
Admittedly, it's no longer unusual for women in their thirties and beyond to shop in the high street stores that used to attract only teenagers. And given that teen and twenty-something fashion is now about looking groomed and glamourous rather than edgy and outrageous, it's not surprising that there's been a certain amount of crossover.
But I'm not convinced that all this inter-generational clothes swapping is a good thing.
That's not because I buy into the idea that no one over thirty should wear a mini skirt or that we should be shopping for elasticated slacks and comfort-fit shoes by the time we draw our pensions.
Yes, there are some items of clothing that tend to look better on girls than women: playsuits, short-shorts and crop tops to name but a few.
But Kylie, Madonna, Carol Vorderman and Helen Mirren have proved that you can be fashion fabulous and get your legs out to great effect whether when you're forty, fifty or even sixty-plus.
And if you've spent years eating healthily, getting regular exercise and slathering on anti-ageing creams, then surely you've earned the right – and the money – to wear whatever you want.
Throngs of yummy mummies have already worked out that the only number that really matters when it comes to fashion is your dress size – and if you're a size 14-16 or below, there's nothing to stop you dressing like a teenager.
Even if you can't squeeze into TopShop skinnies, stores like H&M, New Look and Evans offer exactly the same looks in a wider range of sizes.
So if you feel fantastic in denim cut-offs and platform wedges then wear them with pride, even if you do remember having something similar in the '60s.
No, I'm more bothered by the teenage girls who hanker after the safe, grown-up fashion aimed at women twice their age. What does an 18-year-old want with a classic trench, a pussybow blouse or a pair of Manolos?
We only have to look to Princess Beatrice and her disastrous Royal Wedding outfit to see the catastrophic consequences of dressing like your mother.
Let's not forget that when the teenager was 'born' in the 1950s, girls were on a mission to look as different from their mother's as they possibly could.
Since then there's been a succession of teenage fashions guaranteed to upset parents. From Mods and Rockers, punk, New Romantics, goths, grunge, Madonna-alikes and club kids, every generation has had a uniform that sets them apart from their parents – apart from this one.
Now it's all about sharing your mum's clothes and saving up to buy the latest designer must-have – and where's the fun in that?
So I think it's about time that today's teens left their mums on the high street and took a few style lessons from Lady Gaga.
We need more leotards as daywear, stick-on facial prosthetics and dresses made of meat, otherwise today's teens are never going to be able to look back in horror at the photographic evidence of their fashion mistakes and mums are never going to get the chance to disapprove of their daughter's outfits.
So come on kids, it's about time you started dressing your age – because us grown-ups certainly aren't going to.