"I don't think anyone acts their age any more," says Mr Kenneth Daly, marketing manager for Marks & Spencer Ireland. Mr Daly is "very surprised" because the clothing retailer's new fashion range, designed to entice younger customers into its stores, is attracting interest from a wider age group than expected.

The Per Una range of clothing, footwear and accessories, introduced last month, is intended to give the struggling retail chain a more youthful image in Britain, where Marks & Spencer traditionally caters for an older market. The English chain is trying to shake off its reputation for stocking womenswear that is neither chic enough to compete with more fashion-conscious brands nor cheap enough to challenge some of its bargain-led department store rivals.

"Initially, when I saw the clothes I thought they would predominantly appeal to 18- to 35-year-olds, but the ages have ranged from 20 to 70: it is a very mixed age profile," says Mr Daly.

"It is fashion that is going to appeal to a broad cross-section of people, although there is a definite youth element," he stresses.

"The tailored, formal wear has had a good response from older customers, whereas younger people are buying a lot of the denim."

Marks & Spencer is finding, in its Irish stores at least, that abandoning frumpier fashions need not mean losing its older customers. The performance of the Per Una label, designed by founder of the Next chain Mr George Davies, is one example of how stressing the "definite youth element" to a product can reap its rewards by attracting older consumers too.

"Younger teens have always aspired to act like older teens and so would be aware of communications directed at them," says Mr Gerard O'Neill, managing director of the market research company Amβrach Consulting. "But older adults who want to keep an eye on what is contemporary and fashionable would likewise be interested in messages aimed at the youth market."

These older consumers are referred to as the "grey market" or the "silver market" by advertisers. "People wouldn't want to hear themselves described in these terms but they are sometimes used," says Ms Miriam McDonald, chief executive of the Marketing Institute.

"It is definitely the case that the older market is overlooked," she adds. "A 60-year-old today isn't like a 60-year-old was 40 years ago. People of that age are trying new things, they are travelling and taking up new activities. They are also very discerning and know what they want."

What they want isn't just slippers, sweater-vests and Werther's Original, either.

Ms McDonald points to recent studies on the needs and wants of the 40- to 60-year-old age group.

"Drury Research found that people in this age group who have undergone major changes in their lives - if their husband or wife has died, if they divorce or separate or are made redundant - often have more in common with groups that are 10 years or more younger than them than they do with their peers."

Within this age bracket, Drury Research has identified a number of types of people and how they react to advertising. "Rebels" in their 40s, for example, travel without their children, want to try the latest technological gadgets, and are into e-mails and text messaging. They respond to honesty in advertising and do not buy into "badge brands" as much as younger age groups.

People in their 50s whose children have left home may be "time-rich" leisure seekers who are open to new experiences, whilst other groups may be influenced by marketing messages directed toward teenagers and 20-somethings precisely because they still live with their children.

The research study also found that "golden glories" in their 60s are able to ignore the fact that an ad or a product may be aimed at younger people.

"A lot of people don't want to have the stereotypical image of the older person," agrees Mr Daly at Marks & Spencer. "They want to wear the clothes they like or feel comfortable in, not what they feel they should wear. Lifestyle magazines and lifestyle television programmes have had a big influence, and this has had a bearing on the market in the last two or three years: older people have started wearing younger, fashionable clothes."

According to the Target Group Index, which measures consumption patterns in Britain, as many 25- to 44-year-olds own Reebok and Adidas shoes as 15- to 24-year-olds.

Retailers such as Top Shop, whose official target market is in the region of 17 to 25-year-olds, also attract "significantly older" customers, according to a spokesman for the company.

"We sometimes hear about the phenomenon of mothers and daughters shopping together, and certainly we would have clothes in our TS Design range, for example, that would be well-suited to older customers," he says.

Advertisers often find a different group of people responds to their products from the group they were originally intending to target.

"Johnson's Baby Oil was supposed to be just for babies but when the makers found that women were using it, they changed their marketing strategy and brought out a new product range to target this market," cites Ms McDonald.

Another recent example is the launch of "baby glossy" magazines Cosmo Girl! and Elle Girl in Britain, where publishing companies are capitalising on the fact that younger teenagers buy magazines that are aimed at older age groups. Just 17, for example, has always been read by girls who are just 13 or just 14.

But sometimes attracting the unexpected attention of a different age group is less of a happy marketing accident and more of a headache for companies. In the late 1990s, a worldwide decline in sales of Levi Strauss jeans was attributed to the company's failure to keep up to date with its youth market. It seemed its Levi 501 jeans were more likely to be endorsed by political leaders in their 40s and 50s than combat-wearing rock and pop stars with teenage fan bases. In 1999, the company closed half of its manufacturing plants in North America.

"That's always a problem for young people's fashions. There are a large percentage of people who certainly don't want to wear what their parents wear. A teenager wearing Levi's may not be so happy if their father is also wearing them," says Ms McDonald.

For Levi Strauss, the company's older market was seen to damage the credibility of the brand for trend-spotting teenagers. But in most cases, advertisers specifically target the youth market in order to attract people who have yet to form lifetime loyalties to particular products: in the 18 to 24 age group, Mr O'Neill at Amβrach Consulting points out, advertisers find welcome "customers for life".

Targeting the 18 to 24 market remains vitally important, according to Mr O'Neill, and Amβrach Consulting is currently conducting a study of Irish youth lifestyles and consumption patterns on behalf of the Decode Consortium.

"The nature of the 18 to 24 market is that it is quite fickle and short-term, so there is a need for advertisers to constantly reinvent products for every new generation," he says. "But a generation could be as little as every five years or so."

Advertisers aiming directly for the youth market seem to echo the message of an old Pepsi Cola television ad: "Generation Next! Next phase, next stage, next craze, next wave."