The difference between 18- to 49-year-olds and 25- to 54-year-olds is much more than just a few years.

The conventional wisdom in the advertising and television industries is that 18-to-49 is the "money" demographic; that they're the viewers that advertisers want most to reach because they have disposable income and are not yet locked into product loyalty. They are also the hardest to reach, in that they -- particularly young men -- watch less television.

For years, NBC -- thanks to "Friends" and other youthful comedies -- has owned the 18-to-49 demographic. But now CBS, which steadily has been eating away at NBC's lead among younger viewers, is making the case that slightly older viewers actually are more important to advertisers. Not surprisingly, CBS has long held the lead in the 25-to-54 age group.

As the networks rolled out their 2004-05 prime-time shows to advertisers here this week, CBS and NBC were telling different stories about which age group has the most money to spend and the most willingness to spend it.

Advertisers now pay more for shows that deliver the younger audience -- especially in highly concentrated amounts. Convincing an advertiser that your network brings more younger viewers can mean a swing of hundreds of millions of dollars in advertising money over the course of a television season.

For instance, a 30-second commercial on Fox's "That '70s Show," which rated 28th among 18- to 49-year-old viewers during the past season but is considered by advertisers to reliably bring young viewers, went for $193,000 during the 2003-04 season, according to a February Nielsen Media Research study. But the same 30 seconds on CBS's "CSI: Miami," which rated 10th among 25- to 54-year-olds, brought $185,000.

During CBS's presentation of its prime-time lineup on Wednesday at Carnegie Hall, Leslie Moonves, network president and chief executive, told advertisers that CBS wants to win the younger audience -- with such new shows as "Clubhouse," which follows the life of a teenage batboy for a big-league baseball team.

But "CSI: New York," the most recent spinoff of the franchise that scores big with the 25- to 54-year-old audience, is CBS's way of telling advertisers that older viewers are valuable as well, and perhaps even undervalued.

"I don't know about you, but I haven't met many upscale 18- or even 24-year-olds," Moonves said. "And the ones I have met usually are living in their upscale parents' upstairs bedroom."

Moonves said 18-year-olds aren't likely to buy cars; instead, they're likely to ask their parents to buy them cars. "Trust me; having three teenage children, I know a little something about this," Moonves said, showing the crowd of CBS employees, media buyers and advertisers a picture of him with his three children, "and, yes, I've bought them all cars."

Nielsen data show that 51 percent of the sought-after "young affluent audience" -- 18- to 34-year-olds living in households that make more than $75,000 per year -- are dependent on someone else's money.

The demographic gets poorer as it gets younger, said one advertising buyer.

"The 18-to-24 group doesn't have a lot of discretionary income," said Shari Ann Brill, a media buyer for Carat USA. "They're in school, so [Moonves] does have a point. But that younger [group] is very good for movies and fast food [advertising], though, yes, to a certain extent, it is their parents' money."

NBC, which has seen its lead in 18- to 49-year-old viewers shrink to gains by CBS and Fox, fights back with Nielsen data of its own, showing that NBC still wins both 18-to-49 and 25-to-54 audiences if CBS's 90 million Super Bowl viewers and its other prime-time sports audiences are factored out. Excluding sports, Fox is a close third in younger viewers, with ABC lagging in fourth. Neither network is close to NBC and CBS in the 25-to-54 age group.

During Monday's NBC presentation, NBC Universal Television Group president Jeff Zucker predicted that "Friends" spinoff "Joey" would help the network retain its lead among younger viewers, which he touted as highly valuable.

NBC's big footprint in the 18-to-49 demographic is Donald Trump's "The Apprentice," which Zucker said solidified the hold on young viewers the network feared would be lost with the exit of "Friends."

"We listen to our customers, and they consistently tell us that the 18-to-49 viewers are the most valued audience in television," Randel A. Falco, NBC Universal Television Networks Group president, said in an interview yesterday. "NBC can deliver that demographic better than any network."

Advertisers in search of youth don't always act expectedly. Suppose "Show A" has 5 million viewers ages 18-49 and 1 million 50 and older, for a total of 6 million viewers. "Show B" also has 5 million 18- to 49-year-old viewers but 5 million who are 50 and older, for a total of 10 million viewers.

Common sense would suggest that advertisers should pick "Show B," which delivers the younger audience plus 5 million bonus viewers. But they do not. They pick "Show A" -- and pay more for it -- because a greater percentage of the show's total viewership is younger, said David Poltrack, vice president for research at CBS and UPN, its sister network. This is where NBC has long bested CBS, which always has had more total viewers than NBC.

Consider Fox's "The Simpsons" versus CBS's "Without a Trace." According to Nielsen's most recent ratings, "The Simpsons" ranks 28th among 18- to 49-year-olds. "Without a Trace" ranks 20th in the same age group. Yet, "The Simpsons" demands $278,000 for a 30-second commercial, according to Nielsen's pricing survey, and "Without a Trace" gets only $173,000.

"It's an illogical aspect of the marketplace caused by a lot of people chasing these younger-skewing shows," Poltrack said.