Haven't we seen this plot before?

The trials and tribulations of Millie and co in This Life proved a runaway success. So much so that television keeps trying to repeat the formula. Bored? You will be.

Are you an under-achieving, self-absorbed, middle-class woman in your twenties? Or perhaps a young, northern male with a large chip on your shoulder? How about a frustrated graduate lacking in a career direction? If you happen to fall into any of these categories then you should be glued to your sofa and beaming with pleasure at seeing your life enacted on the small screen tonight.

Are you an under-achieving, self-absorbed, middle-class woman in your twenties? Or perhaps a young, northern male with a large chip on your shoulder? How about a frustrated graduate lacking in a career direction? If you happen to fall into any of these categories then you should be glued to your sofa and beaming with pleasure at seeing your life enacted on the small screen tonight.

Well, perhaps not. But that, presumably, is the thinking behind Metropolis, the umpteenth drama series following the fortunes of a group of university chums transplanted to London to suffer the slings and arrows of adult life.

If present TV schedules are anything to go by, the over-40s are no longer considered important when it comes to evening viewing. How else can you explain the multitude of programmes centred around neurotic young professionals? In the twentysomething bracket we've already had Friends and This Life. Similarly, the over-30s have recently been treated to Cold Feet and Wonderful You. Never ones to tire of an old formula, last night the BBC aired the first installment of Heart and Bones, a seven-part drama which follows the lives of a group of thirtysomething Coventry friends as they - you guessed it - migrate to London. And now we have Metropolis.

The theory is that these are "real life" dramas with which we can all identify. We are supposed to squeal in surprise, perhaps even in horror, as we see shades of ourselves played out on television. Well, to borrow one of Friends' more ubiquitous phrases: "Yeah, whatever."

In TV's "twentysomething" world, graduates usually find themselves catapulted to the big city where they mysteriously have the means to set up home in a small mansion. From there, they fall effortlessly into employment (unless, of course, they are clinging to their anti-capitalist student values: see This Life's Egg, Metropolis's Matthew) and slide smoothly into the bourgeois lifestyle of the criminally over-paid. These people rarely have anything more pressing to think about other than who they are sleeping with, who they wish they were sleeping with or who is sleeping with their girlfriend. Real life? I don't think so.

Never mind the viewers already left out of the equation, ie those younger than 20, or older than 40. You can't help but wonder what the country's actual twenty- and thirtysomethings think of all this. If - and this is a big if - their lives bear any resemblance to those portrayed on television, surely they would be grateful for a bit of escapism in their evening's viewing. And let's not forget those viewers who reside outside London and, heaven forbid, don't work in the media or in the City. You would never visit the capital again for fear of being mobbed by well-to-do whingers and serial shaggers.

Admittedly, the US series Friends gets away with it. Sure, they loll about on squashy sofas all day, never wear the same outfits twice and, since they are rarely able to hold down a job, apparently live off thin air. But the show is also funny, has a strong, sparky script, three-dimensional characters and, crucially, it has a sense of its own ridiculousness.

This Life, though not without its flaws, was also compelling stuff. When it first arrived on our screens it was dubbed the British version of Friends. In fact, it was far grittier than that, full to the brim with casual sex, drug-taking and incurable drunkenness. It had all the addictive properties of a soap - prolonged plotlines, nail-biting cliffhangers - without the accompanying tackiness, and retained the weight of a serious drama; 4.2 million people tuned in to the final episode, where the houseful of lawyers broke up in exceptional scenes of bitching and brawling.

The original mould for these kind of dramas was actually set in the late Eighties with the US series Thirtysomething. It was a highbrow soap-drama where couples juggled their family, work and sex lives to varying degrees of success. As well as observing the torments of their everyday lives, part of its appeal lay in our aversion to certain characters, notably the dream couple Michael and Hope who always had one up on the less functional Elliott and Nancy. The series gave rise to a bizarre new sub-genre and set a formula which writers and directors have been trying to replicate ever since, to decreasing effect.

One of the greatest let-downs in this latest crop of dramas is their spectacularly unimaginative characters. In Hearts and Bones, we have Emma, the weak-willed woman; Robbie, the lager-swilling lad; and Michael, the wealthy investment banker. In Metropolis, the characters are so disparate and tenuously linked that they barely live up to the term "friends". First there's Charlotte, an ambitious financial journalist who is getting increasingly fed up with her armchair socialist boyfriend and seeks solace in the arms of her sexagenarian boss (if the scenario sounds familiar, remember This Life's Millie, the young lawyer who loses interest in her layabout boyfriend and is seduced by her much older boss). There's Alistair, the hopeless dolt without a girlfriend, a job or a roof over his head, and Sophie, a pushy junior researcher for a Conservative MP. The script is littered with toe-curling truisms ("All the promises we made each other on the day we graduated... I think we probably knew they were a dream") and the characters are nothing but caricature.

There is no reason why television shouldn't turn its attention to specific sections of society in its search for new audiences and new relevance. But with every aspiring TV company thinking that they can produce "the next This Life", we are now overrun with flimsy dramas all trawling the same boring themes. Just to hear the synopses of Metropolis and Hearts and Bones is to become overwhelmed by feelings of uninspired déjà vu.

But then, in their increasingly desperate bid to up ratings, television has rarely known where to draw the line. Following the success of ER, inferior hospital dramas are two-a-penny ( Holby City, Always and Everyone, to name just two). And, these days, there's no escaping fly-on-the-wall docu-soaps, cookery shows and home and garden decorating programmes.

Don't be fooled into thinking they'll learn by their mistakes. Television companies are probably working day and night to come up with a game-show to trump Who Wants To Be A Millionaire; no doubt an inferior Ali G will soon be on our screens. As history has repeatedly proved, in the world of television you can never have too much of a good thing.

'Metropolis', tonight, 10.05pm, ITV; 'Hearts and Bones', 9.20pm, Sunday