ho says Shakespeare doesn't matter to young people anymore? On a recent episode of ''Gilmore Girls,'' the hourlong flagship drama of the Warner Brothers television network, Rory Gilmore's high school English class performed scenes from the Bard. Rory, cast as Juliet opposite the handsome and frequently suspended Tristan as Romeo, fretted that the heat of their onstage kiss would expose to her current boyfriend, Dean, the fact that Rory had once kissed Tristan at a party when she and Dean were temporarily broken up. Still, the performance had to be convincing, because it counted for 50 percent of her grade. ''People who fail Shakespeare don't get into Harvard,'' admonished the group's alpha female, named Paris -- who, upon seeing that their rehearsal space has not yet been vacated by an adult aerobics class, remarks, ''What's with the cast from 'Cocoon'?''

Tough stuff for older folks to identify with, to be sure, but we've all had a generation or more to grow accustomed to the fact that, while we ourselves may age, popular culture remains a kind of garden of attenuated youth. And while we may not like it, we all think we understand the reason for it: youth is where the money is. The WB, after all, is not run by a bunch of teenagers bent on self-expression; it's part of a multibillion-dollar entertainment conglomerate whose programming decisions are based on sober business acumen. So if they took the risk of launching a new broadcast network in 1995 -- when the network TV audience overall was shrinking -- then they must have a pretty good idea of who that audience is and what it wants. Right?

Well, if you assume that a TV show's ''audience'' consists of the people who watch it when it's on, your first conclusion might be that the folks at the WB are laboring under a gross misconception. Nielsen ratings for ''Gilmore Girls,'' when considered as raw numbers, are horrible. Somewhere around five million people watch an average episode, which puts it in 121st place among the 158 shows broadcast in prime time this past season.

But if you consider that a TV network's true audience is advertisers, then you're on your way to understanding why Tuesday night is, in fact, a big moneymaker for the WB. The network more than makes up for its abysmal ratings by charging an inflated ad rate for those few viewers its shows manage to attract. A 30-second commercial spot on ''Gilmore Girls'' costs about $82,000 -- nearly threequarters of the fee for advertising on an episode of, say, ''Law and Order: SVU,'' an NBC program that regularly has about three times the number of viewers. The WB gets away with this because its overall ratings, poor as they appear, were up 5 percent in the 18-to-34-year-old category last season, and while ''Gilmore Girls'' may be among the least-watched series on television, it's also No. 2 in its time slot among viewers aged 18 to 34.

Eighteen to thirty-four: for decades, conventional advertising wisdom has attached the adjective ''coveted'' to this slice of the viewing audience. According to an analysis by the former NBC News president Lawrence K. Grossman, advertisers pay an average of $23.54 to reach 1,000 viewers in that age bracket, versus $9.57 per 1,000 over the age of 35. And since commercial television, whatever else it may be, is fundamentally a system for delivering audiences to advertisers, network executives lose a lot of sleep trying to figure out what will hold fast the slippery attention of people in their late teens, 20's and early 30's. It is, as it has been for 40 years, the principle by which a great deal of our popular culture -- not just TV, but music, movies, radio -- comes into existence.

The odd thing is, there's no real reason for it anymore.

People over the age of 50 account for half of all the discretionary spending in the United States. Proportionally speaking, there are more of them than there ever were, and they are voracious cultural consumers. They watch more television, go to more movies and buy more CD's than young people do. Yet Americans over 50 are the focus of less than 10 percent of the advertising.

What makes advertising an entertaining field of study is that its twin natures -- pop art and dismal science -- are never really reconciled. If the notion of the ''target demographic'' lives on well past the point where it stopped obeying any kind of economic logic, it may be worth wondering how much sound, unsentimental business sense was ever behind this juggernaut to begin with.

rand loyalty: this was the concept that turned the minds of young people into an advertising battleground, before tele- vision was even invented. Get them early, the thinking went, and if your product isn't junk, then you'll have that customer's fidelity for life. And, of course, advertising aimed at the young has always had a secondary target as well: those who aren't young but want to appear so, who believe that purchased commodities have the power to stave off time.

But in the earliest days of television, when the popularity of network programming was measured mostly by the sales of TV sets themselves, there was no question of ''targeting'' anything but the broadest possible audience. It wasn't until the 1950's that the A.C. Nielsen Company started breaking down its crude data on the TV audience by age as well as income, geography and other categories -- at which point advertisers began to develop more of an interest in some TV viewers than in others.

''Embedded within the 18-to-34 cliche is a lot of social and economic history,'' says Stuart Ewen, author of ''Captains of Consciousness'' and several other books on the history of advertising. ''The development of that group coincided with the dramatic expansion of the American middle class in the years right after World War II. The notion was that these young people coming out of the war were going to be the engine that drove the American economy.''

It would be giving advertisers of the late 40's and 50's too much credit, though, to say that they got onto the demographic bandwagon right away. The work of such Eisenhower-era ad barons as Rosser Reeves and David Ogilvy relied almost smugly on simplicity and repetition, on what Reeves termed the Unique Selling Proposition drilled mercilessly into the public consciousness: ''Wonder Bread Helps Build Strong Bodies 12 Ways,'' ''Pepsi Refreshes Without Filling'' and so on. Indeed, Reeves's famous Anacin ad featuring an animated hammer pounding inside one's head could function as a metaphor for both the intent and the effect of late-50's advertising in general. The very idea of targeting some demographic niche would have been unknown to Reeves; his own ad-spending mantra was characteristically drab and concise: ''the most people at the lowest possible cost.''

By 1960, though, when Bill Bernbach, the man generally credited as the father of Madison Avenue's ''creative revolution,'' placed a photo of a Volkswagen just above the large-type word ''Lemon'' (an event that had roughly the effect on advertising that the 1913 Armory Show had on the history of American art), the pendulum had begun its long swing from paternalistic notions of brand loyalty to exuberant iconoclasm. The advertising industry ushered in its own version of the Age of Aquarius, in which youthfulness -- being young, thinking young, speaking young, buying young -- was all.

To be sure, there was a hard-numbers aspect to the initial explosion of youth-targeted advertising in the 1960's and early 70's. By 1966, 48 percent of the U.S. population was under the age of 25. Failure to speak their language meant kissing off half of the market.

Still, this is advertising, in which numbers never tell the whole story. Thomas Frank, in his brilliant study of 60's advertising, ''The Conquest of Cool,'' offers the example of automobile ads; in the 40's and 50's they preached reliability and endurance (a typical ad might picture a happy nuclear family out for a Sunday drive), but in the 1960's they suddenly aspired to the symbolism of revolution: Oldsmobiles were rechristened ''Youngmobiles,'' consumers were exhorted to join the ''Dodge Rebellion'' and as staid a make as Buick promised consumers ''Now We're Talking Your Language.'' This all seems understandable enough in the context of the times, until you consider that in the mid-60's young adults accounted for only 9 percent of all new car sales. So why would the car business bother to target them?

The business world, it seems, was going through its own generational insurgency, and the old model of customer relations was tossed gaily out the window. In a society in which young people predominated numerically and were acknowledged as the vanguard of change, the idea of brand loyalty was turned upside down. What advertisers prized in American young people was their disloyalty, their insistence upon the new. The notion of ''revolution'' (i.e., fashion) could be applied to any and every commodity -- and common sense was no obstacle: when Pepsi adopted its wholly metaphorical slogan ''Join the Pepsi People Feelin' Free,'' sales soared.

Such ads were never designed to extract riches from the nation's youth -- ''youth'' simply became their new subject matter. They posited a plain-speaking friendship between the advertisers and the young, a friendship that was entirely fictional but seemed really cool, and the way to get in on it was to purchase the product being advertised. The genuine counterculture was, of course, tiny in comparison to the legions of people who admired it and wanted to be a part of it in some small, risk-free way -- by, for instance, joining the Pepsi Generation. It was a seductively undemanding model, for advertisers and consumers alike, and it kept the business world's focus squarely on the 18-to-34 bull's-eye for the next three decades.

How did the TV networks satisfy their advertisers' demand for this newly calibrated audience? Well, if they couldn't always bridge the gap between themselves and the bona fide counterculture, they could certainly attract the attention of those who wanted at least to feel that they could lay some claim to membership in it by watching TV.

''That period -- the end of the 60's, the beginning of the 70's -- was really an extraordinary moment in our culture,'' says Robert Thompson, director of the Center for the Study of Popular Television at Syracuse University. ''In one fell swoop, CBS canceled a whole bunch of programs that were still fairly high-rated -- 'Mayberry, R.F.D.,' 'Hee Haw,' 'Gomer Pyle' -- and replaced them with a very different kind of show.''

The highest-rated show for the 1970-71 season was the decidedly unrevolutionary ''Marcus Welby, M.D.''; No. 2, though, was ''The Flip Wilson Show,'' and other programs like ''The Mod Squad'' and ''Rowan and Martin's Laugh-In'' sneaked into the Top 20. These hybrids of old forms (sketch comedy, the cop show, the family sitcom) and young subject matter became the entree into the cycles of hip for millions of viewers, and the advertisers who paid for those programs gave their viewers a way to make the idea of permanent revolution not just a philosophy but something they could take home and put on a shelf, or in their closet, or in the fridge.

''TV definitely became more research-driven and more demographically self-conscious in the 60's and 70's,'' says Mark Crispin Miller, director of the Project on Media Ownership at New York University. ''I think one can safely say that entertainment generally is research-driven now, but television, being the most directly responsible to advertisers, was the first to take the plunge.''

The first, but not the last. Hollywood discovered, somewhere around the release of ''Star Wars,'' that movies could also profit by functioning as advertisements for their own merchandise; whereupon they, too, started pitching their work to a younger audience. Commercial radio, determined to lead rather than follow music's fruit-fly-like cycles, undertook the ghettoization of programming intended for anyone above the age of 29. Thus the cultural productions of what Variety magazine, in its inimitable style, calls the Zitgeist continued to snowball. By the early 80's it had grown into the self-fulfilling prophecy (Why are movies designed to appeal to people in their teens and 20's? Because those are the people who go to the movies.) that we're still living with today.

It was a long time before anyone cared to notice that the target demographic itself, and its status in American society, had gone through some profound changes. The population bubble caused by the baby boom kept floating up; whereas in 1940 only 6.8 percent of the population was 65 or older, as of 2000 that number was 12.4 percent. And the economic news wasn't bullish either. Between 1973 and 1990, median real income for families with children headed by persons under 30 fell an amazing 16 percent. And in 1990, three out of four men between the ages of 18 and 24 were still living at home, the largest proportion since the Depression.

''Young people's hopes and prospects for the future have in very real terms become diminished,'' says Stuart Ewen, ''and in a situation like that, obviously you have to rethink whom you're selling to.''

And what of the theory of brand loyalty -- the idea that winning over the young consumer means winning him or her over for life? There the big change has come about not so much in the young but in the old. The baby-boom generation, raised in front of the TV, just never became brand-loyal in the way their parents were. Everyone is pretty malleable these days: 67 percent of female heads of household between 18 and 34 were willing, in a Nielsen study, to try a new brand even if it went against their customary buying habits; the corresponding number in the 35-to-64 age bracket was 70 percent.

Even the argument that most pop culture is for young people because young people consume the most pop culture has begun to fall apart. ESPN's highly promoted X Games, a kind of ''alternative'' Olympics featuring skateboarders, BMXers and the like (referred to by The Wall Street Journal as ''the Holy Grail of youth marketing'') was outperformed this summer on the network's primary channel by the bargain-basement Great Outdoor Games, a decidedly non-youth-oriented event featuring lumberjack contests and the talents of various sporting dogs. Over the last decade, the proportion of the national moviegoing audience between the ages of 50 and 59 doubled, while the proportion of teenagers shrank steadily. The percentage of CD's sold to consumers over 45 doubled as well.

And yet the romanticization of youth persists: the adjective ''coveted'' has been joined by the phrase ''hard to reach'' as a justification for the premium advertisers continue to pay to speak to the 18-to-34 crowd. Put aside for the moment the fact that these so-called hard-to-reach young adults spend an awful lot of time with the TV on -- men between 18 and 24 watch more than 20 hours a week, according to the Nielsen people; put aside the fact that those young X Games rebels come plastered head to toe with corporate logos. What logic suggests that, because there are proportionally fewer young people than there used to be, because they have less money than they used to and because it's harder to separate them from that money than ever, advertisers should spend more money trying to court them? It would make as much sense to say that advertisers really ought to pay top dollar for viewers who don't have any spending money at all.

If you ask the agencies themselves about the relevance of the target demographic, they're likely to tell you that numbers-oriented research of any kind is so last year. Forty years after creative advertising's Big Bang, the study of demographics is a ''science'' many now scorn as outdated and crude. ''Now they call it psychographics,'' Thomas Frank says. ''They hire sociologists, anthropologists -- it's very elaborate.'' The methodology of today's market research often approaches the mystical.

So who's willing to pay the WB extra to reach today's young adults? The ads featured on ''Gilmore Girls'' themselves paint a portrait of the coveted youth audience. Apparently, they spend as if they still get an allowance. Wendy's , Snickers, Cover Girl makeup, chocolate milk -- there was hardly a product advertised on ''Gilmore Girls'' that would cost a consumer more than $10. With one glaring exception: new cars. Ford and Honda advertised throughout the Tuesday-night lineup.

''These younger folks may not be big-ticket purchasers now,'' says a Ford spokesperson, ''but they may one day be. Ford wants to form a relationship with these younger buyers now and grow them up into our various brands.'' As for Honda, it has, according to a company representative, ''pretty much one of the youngest buying demographics of any car company out there. The Civic in particular -- almost all the ads on the WB are for Civics. And we're on MTV all the time.''

And how many of these youth-oriented Civics, sticker-priced at a minimum of $14,000, are actually sold to people under the age of 26? One in five. Not so different from the 60's.

They'll catch on eventually. But advertising is a vast mechanism, risk-averse and inertia-driven, and like most multibillion-dollar industries it changes course with all the agility of an oil tanker. And so, for now, the polestar of the target demographic endures. It has gone from an ecstatic confluence of societal change and economic opportunity to a fusty business institution.

Of course, it's more than that as well. No matter how many dollars might be squandered in the process, you see in modern TV advertising what you see in, say, Greek statuary: a cultural key, a worldview whose increasing irrelevance to cold economic models only testifies to how compelling it remains for us.

In the meantime, the Fox network, eager to reassure advertisers made restless by its drop last year to second place among 18-to-34-year-olds, has just announced that this fall it will become ''bold, younger, more noisy.'' The network's new motto? ''It's Good to Be Bad.''

Jonathan Dee is the author, most recently, of the novel ''Palladio.''