Age does not weary them and at the going down of the sun they are ready to party. The over-fifties are the richest group in Britain, holding 80% of private wealth, and they are not averse to spending it.

Yet one of the great mysteries is why this vast market, worth £260 billion in disposable income, has been almost ignored by retailers and advertisers who prefer to target young people with only a fraction of the spending power.

The over-fifties, after all, buy more Porsches and rock music than people under 30. They have embraced computer games with the passion of youngsters and prefer scuba diving on exotic holidays to babysitting their grandchildren. Their zest for health, fitness and prolonging youthful looks has earned them the label “recycled teenagers”.

Professor Richard Scase, a government adviser and futurologist, says: “This is a hedonistic category of people who say, ‘Spend, spend, spend. We’ve had 20 years of hard slog and now we’re going to enjoy ourselves’.”

Scase confesses that he is not immune to the impulse: “I’m over 50, I was in a wine bar last night, I drive a soft-top car and my partner is 24. My behaviour is not at all like someone in their fifties, but that goes for a lot of people.”

Conventional wisdom is that older consumers tend to stick with their brand choices and resist change. But Scase’s research shows that age is no longer a predicter of people’s consumer behaviour or their attitudes. Ten years ago the young were more inclined to experiment with new ideas and products. Now the over-fifties have taken the lead.

“It’s to do with extensive life expectancy,” he believes. “Twenty years ago, when we were an industrial economy, people expected to live into their late sixties. Now it’s 80 and there’s a total reassessment of what people want out of life.”

The contortions undertaken by the advertising industry to avoid images of middle-aged people are many and varied. Brands such as Honda and Toyota are more popular among older consumers because of their reliability, but their advertisements dare not admit it.

Mature women find resonance with the faces of such beauties as Andie McDowell and Catherine Deneuve but, perplexingly, Isabella Rossellini was dropped by Lancôme at 42, deemed too old. Guinness, desperate to get young people to buy its product, is taunted by the fact that one of its “coolest” advertisements starred an ancient Italian swimming champion.

So why do advertisers turn a blind eye to the over-fifties who make up 44% of the population? Scase contends that inertia has preserved the outdated age categories that guide advertisers. The first of these, aged 16-24, retains the old assumption that women have their first child at 22 or 23, which is at odds with today’s reality.

“I think a huge amount of advertising spend is wasted because it misses the point,” he says. “In the past your 45-plus person was someone who was settled, married and wore slippers. That’s all broken down in a society where 40% of all households are single person.”

Divorce represents another missed marketing opportunity — the number of people in their fifties searching for a new companion. “They have to focus on their appearance and fashion because they can’t take their partners for granted,” says Scase.

A more chilling explanation for advertising’s neglect is offered by Mark Wnek, former chairman of Euro RSCG, Britain’s second largest communications group. “Basically, all young people want to get on and climb over the bodies of old people. The sooner you can call someone old, the quicker you can climb over them. And the age of ‘an old bastard’ is coming down and down,” Wnek says.