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Want younger viewers for the evening news? Ditch the geezer ads, says “ABC World News” anchor Charlie Gibson.

If networks are serious about luring pre-Social Security eyeballs, they should replace commercials for adult diapers, dentures and “patent medicines” with spots for younger, sexier products, in Gibson’s view.

The current commercials “bespeak an older audience,” says Gibson, in Philadelphia last week to anchor “World News” from the National Constitution Center. “I’d rather have car ads.

“When you put on ads mostly for medicines, you’re saying, `We want an older audience.’ I would like ads that say, `We have a younger audience here.”‘

Not likely, Chas.

Median age of “World News” viewers this season is 59.9, according to Nielsen Media Research. It’s 60.3 for Brian Williams’ No. 1 “NBC Nightly News,” and 59.5 for Katie Couric’s “CBS Evening News.”

Networks target 25-to-54-year-olds for news.

At 63, Gibson is nine years out of the demo. And he’s a first-time grandpa, to boot. (Daughter Jessica had a son in March.) Couric is 49, two years older than Williams.

“Why say the important audience is 25 to 54 if you run ads that obviously appeal to people over 60 or 65?” Gibson says. “Why aren’t you trying to get commercials that appeal to the audience you want to get?”

So why did Gibson veto a recent guest shot on Stephen Colbert’s hip-a-doodle-do “Colbert Report” on Comedy Central?

“I just didn’t think it was something I should do. I wouldn’t feel comfortable.” Jon Stewart’s “Daily Show” hasn’t asked, Gibson says, adding that both hosts are “brilliant.”

Hard to believe, but Colbert was a correspondent for “Good Morning America” for about 12 seconds in the mid-’90s.

Gibson, “GMA” co-anchor at the time, says he didn’t even realize Colbert was on the show until he saw a clip with both of them on a “60 Minutes” profile of Colbert on April 30.

“Apparently, he did one piece for us,” Gibson says. “I have no memory of that. None. Apparently I interviewed him on the set. He was on a monitor.”

As for letting Colbert slip away, Gibson says: “Boy, did we screw up. He’s a real talent. It would have been good to have him as part of `GMA.”‘ Back then, maybe. Now, of course, his act would never play on broadcast TV.

Unlike his competitors, Gibson didn’t exactly slide into the anchor seat.

After not getting the job in January, he had decided to retire in June 2007. Then co-anchors Bob Woodruff got injured in Iraq and Elizabeth Vargas became pregnant, and Gibson was named solo anchor May 23.

The situation is not without irony. Gibson will anchor the 2008 presidential elections — the sticking point in January. ABC News boss David Westin wanted him out in June 2008, network insiders say.

“I had no bitterness when it didn’t work out,” Gibson says. “I understood what they were doing and why they did it, and, indeed, defended it.”

Contrary to a published report, Gibson swears he never threatened to quit if Westin didn’t give him the throne the second time. “Never happened. Scout’s honor.”

He’s far more equivocal about how long he’ll stick around, however.

“I suppose it’s conditional on how I feel, and on those horrible things called ratings. Whether they like the cut of your jib doing this.” (Cut of your jib? Spoken like a Princeton man.)

Gibson says “World News” will continue as “a very straight news show.” No bells, whistles or, like CBS, “Free Speech” commentaries. “We’ve got 22 minutes. That’s precious time.”

He still considers the Big 3 the gold standard of TV news. He doesn’t watch much cable, calling it “a lot of people yelling at each other.”

ABC execs are quieter when Gibson shares his theory on “World News” ads. “They smile and pat me on the head,” he says. “They think my business is doing the show and their business is selling the ads.”