Joan Smith: Sam Cam up till midnight! Outrageous!

Once again it was Twitter that broke the big story: Sam Cam went to a rave in Ibiza last weekend. She was spotted by a British user of the social networking site who spoiled her coup – that rare occasion when the words "rave" and "Ibiza" and "prime minister's wife" could legitimately appear in the same sentence – by commenting that it was funny to see Mrs Cameron among so many "young ravers".

Oh, come on. Mrs Cameron is 40 and it hardly takes much energy to stay until the end of an event which apparently finished at ... midnight. I mean, what's happened to Ibiza? I go to dinner parties in London that end later than that.

When I had a little drinks party last winter, people slept in every room of the house and the last guests left after breakfast the following morning. I secretly hope Sam Cam was just limbering up on Friday night, getting ready to move on to the real party while the twentysomethings stretched, yawned and went to bed.

All right, I know Norma Major didn't go to raves. I can't think of another prime minister's spouse in history who's queued for a budget flight to Ibiza, checked out the scene and joined a crowd of thousands to watch the dance DJ Pete Tong, although I suppose Denis Thatcher might have gone to Spain for the golf. It's a generational thing, and Norma's husband John Major just happens to be the last British prime minister born before the baby boomers came along and changed (almost) all the rules.

Gordon Brown is technically a baby boomer but it's even harder to picture him at a rave than it is Disraeli, who did at least have a bit of louche style. Brown even went on holiday in a suit, looking as though he was desperate for an international crisis to erupt so he could get back to the office.

I know we all hate Tony Blair but one of Brown's misfortunes was that his predecessor created a new style of prime minister, relaxed and informal as well as (in the end) widely loathed. You may not want to be reminded of this but back in 1997 the Blairs were youthful and almost cool, filling 10 Downing Street with pop singers and soap stars.

The Camerons are the next generation and there's no denying they're the baby boomers' cultural heirs. They don't share the centre-left politics – I guess it's hard if you're that rich – but they've absorbed the rest of our assumptions wholesale. Age-appropriate behaviour, what's that? The youngest of Sam Cam's children is less than a year old but that doesn't mean the PM's wife can't go to parties and stay out really, really late, sometimes coming home after midnight!

The astonishing thing is how long it's taking the rest of the world to catch up. I'm forever reading that this actress still looks amazing at 50 or that TV star turned up in wellies at Glastonbury, even though he's entitled to a bus pass. So what? The seven ages of man (or woman) always seemed a pretty crap notion to me, involving giving up most of the things you liked – rock 'n' roll, fashion, sex, parties – without getting much in return.

I used to love Blondie, now I listen to Lady Gaga and I'm not going to pretend I'm suddenly a big fan of Mahler. In this world that knows the rules have changed but hasn't got used to it, Sam Cam is at a dangerous age. When her husband joined her at the weekend, she stepped out in a blue halter-neck dress that showed off her upper arms; there's almost a fatwa on bare arms if you're a woman over the age of 29, but that's something else no one's told the PM's wife. Obviously she doesn't think she's too old to have fun – and that's just about the only thing I can ever imagine agreeing on with a Tory.

www.politicalblonde.com