By the time you read this, Fox News may have already fired John Gibson, co-host of “The Big Story with Gibson and Nauert” (4 p.m., Fox News). For their sake, I wish they would.

Last week, as news was breaking of actor Heath Ledger’s death, Gibson used the occasion to mock the actor, call him a “weirdo” and use clips from “Brokeback Mountain” to make cheap jokes about him.

John Gibson has a constitutional right to say whatever he wants and to be a bigoted old fool as he chooses. His odious “war on Christmas” has long exposed him as such.

But Gibson’s comments expose a problem for Fox News that transcends homophobia, cultural politics and mere respect for human decency. For all of its swagger, Fox News has a deadly demographic problem. Its cable news and talk shows are frequently the most watched in prime time, but its audience skews old. “The O’Reilly Factor” (7 p.m., Fox News) almost always trounces its competition in total viewers, but often fewer than 20 percent of its viewers are under 50.

It doesn’t help Fox News when its talent ridicules the young.

It’s only natural that Ledger’s death at only 28 is having a very powerful effect on young people. For some, it may be the first time they’ve come to grips with the sudden death of someone their own age. So Gibson’s remarks are not merely insulting to Ledger but are a slap in the face to a generation that feels particularly stung by the tragedy.

Gibson’s callous comments recall the stupid jokes made last year by Don Imus about the Rutgers women’s basketball team. While much was made about the racial component of that ugly incident, it can also be seen in generational terms. The image of Imus, a wrinkled figure under a cowboy hat saying dreadful things about college students in their hour of triumph was a perverse role reversal. A 67-year-old was acting immature, while college students stood in quiet dignity. Is it any wonder why most decent people sided with the students?

It’s not hard to see how these comments by Imus and now Gibson repel younger viewers and the TV advertisers who covet them. And Fox management must know this. That’s why I expect them to show Gibson the door.

¢ “Nova” (7 p.m., PBS, check local listings) presents “Secrets of the Parthenon,” a look at how efforts to restore the 2,500-year-old Greek temple have unlocked secrets about the genius behind its design and construction. It’s interesting to note that the temple’s restoration, employing 21st-century technology, has taken more than 32 years. The Greeks built the Parthenon in only nine years – more than 400 years before the time of Christ.

Tonight’s other highlights

¢ Auditions continue on “American Idol” (7 p.m., Fox).

¢ A mother’s paralysis may be linked to her daughter’s injury on “House” (8 p.m., Fox).

¢ A conspiracy divides the team on “The Unit” (8 p.m., CBS).

¢ A Navy pilot tries desperately to hide his fears on “In Treatment” (8:30 p.m., HBO).

¢ Scheduled on “48 Hours Mystery” (9 p.m., CBS): A meek wife and teacher becomes a killer.

¢ A mentally challenged girl may be the only witness to an attack on her mother on “Law & Order: Special Victims Unit” (9 p.m., NBC).

¢ An officer needs a good lawyer after defying “don’t ask, don’t tell” on “Boston Legal” (9 p.m., ABC).

¢ Gina’s death rattles Christian on “Nip/Tuck” (9 p.m., FX).

Cult choice

A troubled couple (Alan Cumming and Jennifer Jason Leigh) reunite for a celebration in the 2001 drama “The Anniv-ersary Party” (8:05 p.m., IFC).