Madonna can get away with most things, but copying the likes of Bieber, Rihanna and Miley Cyrus with the golden teeth grill isn't one of them. It's time for the pop icon to grow up, says Sarah Vine

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I've always admired Madonna. Over the years she's kept me entertained with her endless talent for exhibitionism, supplemented by the occasional bit of quite good music. The older she gets, the harder she works at it: the toyboys, the eye-popping biceps, triceps and thighceps, the ridiculous dance moves, her apparent allergy to skirts. So what if she dresses like a drunken teenager, I've always thought: when you're the Queen Mum of pop, it comes with the territory.

But there is something about recent snaps of her that makes even me think the time has finally come for Madonna to start acting like a grown-up. It's not that she's too old to have fun or behave outrageously (there's no age limit on that sort of thing); it's just that she no longer looks like a trend-setting mould-breaker; she just looks a middle-aged woman trying desperately to keep up.

Exhibit a: Her 55th birthday party in the South of France. Firstly, it was fancy dress, which is always a sign of someone that has run out of ideas for having fun. Judging by the photographs, the theme was Marie-Antoinette-meets-Rocky-Horror. Cue an assemblage of extravagant wigs, corsetry and young men in suspender belts and pearl earrings. The star herself spent the night in a heart-shaped eye-patch, which was probably meant to look sexy and mysterious, but in fact just had me wondering whether it was concealing a secret reading glass.

Exhibit b: Her teeth grills. I can just about forgive the likes of Rihanna and Justin Bieber for wasting money on exaggerated gold braces that make the wearer look like they've got advanced tooth rot (was it Mick Jagger who had an emerald removed from his front teeth because people kept telling him he'd got spinach in his teeth?).

The young do stupid things when they've got too much money. But Madonna should really know better: at her age tooth grills don't look "gangsta". They make her look like she's auditioning for a remake of Silence of the Lambs. Either that, or she's forgotten to clean her dentures.