NINE o'clock in the evening, and the office is deserted - except for Madeleine, who has stayed late three nights running to get through a backlog of paperwork. The extra hours she's putting in mean an important client's deadline will be met.

As she adds the finishing touches to her report, her satisfaction is mixed with smugness - her boss's nanny is sick, he has been rushing off early to collect his children from school, and she knows that there is no way his part of the project will be completed on time.

Madeleine is 24. She graduated in business studies six months ago, and she is the most junior in the office. But not for long. What she lacks in experience she makes up for in energy and confidence. Her sights are firmly set on her boss's job, and she has unobtrusively made sure that the managing director is aware of her efforts.

Madeleine's display of cheerful efficiency makes her boss uneasy. But what can he do? His salary costs the company nearly three times as much as hers. He has extensive commitments outside work and can't put in the hours that she can. He has to watch helplessly as she powers her way up the office hierarchy. The wretched girl even handles the computer as if she grew up with one (which she did).

Being traded in for a younger model is an occupational hazard in many jobs. In some fields, youth is relative - in last week's Cabinet reshuffle, John Major booted out a clutch of fiftysomethings and replaced them with a zippy bunch of new bloods on average 10 years younger. But in many professions, the office eager beaver is getting younger and younger.

Graduate job vacancies have increased for the first time in four years as firms take on highly qualified junior staff - ambitious youngsters who are snapping ever more eagerly at the heels of those above them.

Employing feisty young pups has many advantages, according to Tunde Johnson of the Federation of Recruitment and Employment Services. 'The young are computer literate. Over-35s are often lost in terms of new technology. They also have more responsibilities and are less flexible.

'Young people are less demanding - they see any job as the first rung of the career ladder and are willing to do virtually anything. And it's cheaper to take on a young person and train them up. For example, solicitors lay off at partner level and take on graduates. It's an employer's market, they can pick and choose. Even 35 is too old now in some jobs.'

Endless bouncy twentysomething enthusiasm is one trait that may particularly exasperate jaded, cynical old thirtysomethings. 'Young people are hungry and ambitious,' says Ian Lawson, associate director of the Industrial Society, who has noticed a number of very young delegates at the society's prestigious course for high-flying managers. 'They bring freshness to a company - they aren't hampered by remembering ways that things used to be done. Apart from the cost benefit, they bring new ideas - they're too naive to know that something 'can't be done'.'

He also believes job scarcity has done much to sharpen up graduate ambition. 'When I graduated 15 years ago, any decent graduate would get a decent job - we didn't think seriously about work and careers. Today's graduates are much more mature, more tuned in to the real world.'

Tough kids are also less likely to buckle under stress, says Ian Campbell, sales and marketing director of outplacement company Focus. 'Young people have got more energy. My son became a retailing trainee at 23 and was deputy manager in two years, but it was very high-pressure work - never two days off together, weekend working, antisocial hours. I think young people can withstand that rather longer than older ones.'

Economy is also a big factor. 'It's better to recruit a younger person than to retrain an older one,' adds Campbell. 'To be brutal: why keep someone on 30K or 40K when you can go out and get a graduate for 15K?' ( pounds 15,000 is an average graduate starting salary in London; elsewhere this drops to pounds 13,500.)

'Anybody over 30 begins to look too expensive to head teachers and governing bodies,' complains Brian Clegg of the National Association of Schoolmasters/Union of Women Teachers. Teachers' pay is strictly tied to age and experience; a raw recruit with a good degree starts on pounds 12,999 which goes up to pounds 19,614 with seven years experience. 'Teachers can apply for job after job without being taken on, especially women returning to work after a career break. It's terribly frustrating. Schools are only concerned with finding the cheapest of everything - employing someone to stand in front of the kids in the classroom.'

Fresh-faced but predatory office babes can spread alarm and despondency. Middle manager James, 38, feels sadly inadequate compared to his whizz- kid assistant, 12 years younger. 'There's nothing I can really object to. You can't criticise someone for working too hard. But things barely hit my desk before they disappear into his 'Current Projects' file. I like to leave the office by seven at the latest to be home before my children go to bed - it's nice for kids to be reminded of what their dad looks like. But he has no kids, no dog, no bloody barbecue weekends or lawn to mow - he exists on Marks & Spencer canelloni for one and thrives on work, work, work.'

'I can only hope that Mr Time and Efficiency's clockwork will run down if he ever takes enough time to find some outside interests, maybe even one day starts a family himself. Whether that will ever happen is dubious, though, I don't think he's even got a girlfriend. I suppose he might bump into someone at an Information Technology Development Seminar or something. Their eyes could meet over a crackling screen. In the meantime I have to say that he is a delightfully energetic and charming young man who will go a long way, and if he were to disappear under a bus I would shed no tears at all.'

Catherine, 27, newly qualified as a solicitor, makes no bones about the fact that she is out for promotion as quickly as possible. 'I give 110 per cent. I love answering letters by return, keeping on top of things, knowing I'm the most efficient one in the office.

'I feel gleeful when my boss is ill and has to lose time at home, or has to spend an afternoon at the school sports day or when one of her brats goes down with chickenpox and wants its mummy. And why not?This firm works me very hard indeed and I want the rewards - if not this year, then next year. It took me a lot of time and effort and money to get this job and now I have, my willingness and ability to devote myself to work is one of my trump cards.'

However, no matter how personally efficient they are, when the Younger Models shoot up the ladder over the heads of their colleagues, they may find that they have made few friends along the way. 'People who are older than you expect to be more highly paid and have more responsibility, so when that situation is turned on its head, there are bound to be problems,' says Angela, 22, recently promoted above a male colleague nine years older than her. 'It makes me feel uncomfortable too, but I hope that is not a reflection on me, but on the attempts of this . . . person . . . to make the situation difficult.

'He subtly challenges my decisions, so that I have to justify them to him when I ask him to do something. It's sort of 'why should I?' and that's incredibly annoying. Then he starts quoting what it is and isn't his job to do, which is rubbish. I know that no job description has ever been imposed upon his position, because I used to be in it] He is ambitious, and it must rankle that somebody so much younger has been employed in a senior position. I try to be pleasant and polite, but I suspect that one day I'll explode with rage and there'll be a very nasty scene.'

'I spent a term as acting head of department,' recalls maths teacher Caroline, 28. 'This vile old bag who must be 25 years older than me set out to make my life a misery. At her age you'd think she'd be bloody well old enough to know better. Department meetings would end with us both politely snarling at each other through fixed smiles. It was always her trying to tell me that she knew better, had more experience, phrases like 'Well, my dear, you won't remember the days when we used to do this or do that.' My dear, indeed] I refused to lose my cool, though I often ached to give her a good old poke in the eye.'

Resentment can be a problem, agrees Ian Lawson of the Industrial Society. 'There is often prejudice in older minds - but when the younger manager expects problems, it becomes a self-fulfilling prophecy. People respond to being asked for help - nothing switches them on more. Young managers may feel they will lose face if they ask for help, but people like to feel their experience is valued. If you are good at what you do and treat people with resepct they will overcome their prejudices. Though with some people it takes longer than others.'

He believes that anyone who feels threatened by a junior hot on their heels may have other reasons for insecurity. 'We all only achieve anything by working with other people, and any manager worth their salt should want good people on their teams - people who are as good or even potentially better than they are themselves.'

Will babe whizz-kids finally be able to relax once they get their feet under the boss's desk and start raking in the boss's salary? Probably not. Once the drive to work hard and succeed has begun, it seems the pressure to achieve does not let up - particularly if, as Ian Campbell of Focus suggests, the entire structure of working life is changing, and career preoccupations are claiming an ever more central role in young people's lives.

'It would be ridiculous for a firm to sacrifice all older personnel because they would be sacrificing experience,' he says. 'In an ideal world each firm needs a blend of age groups. But the technological revolution has changed working patterns for good.

'The idea of a job, or even a career, for life has gone. As a result, people who are in their twenties now have been changed - and they will be the same when they reach their thirties and forties. Instead of working to live, they have started living to work.'

(Photograph omitted)