David Letterman says he's retiring – what now?

Jimmy Fallon's fast start replacing Jay Leno on the "Tonight" show the past two months had a secondary effect: David Letterman suddenly seemed old.

The Top 10 list, the ironic detachment, even the set at the Ed Sullivan Theater. Time doesn't stop for comedy legends, or superstars of any sort. Letterman, who announced Thursday that he will retire from late-night television sometime in 2015, had to feel it.

CBS now faces the challenge of moving on in a reordered late-night world at a time the two Jimmys – NBC's Fallon and ABC's Kimmel – have a significant head start.

When Jay Leno left in February, Letterman lost his foil – the man whose victory in the competition to replace Johnny Carson two decades ago he never let go. Leno was someone who spoke his language, though, a generational compadre, and when he left, Letterman was alone.

Fallon and Kimmel have a different style, more good-natured and less mocking of the entire concept of a talk show.

It's hard to know what role the new competition played in Letterman's decision. His last contract extension, signed before Fallon took over, was for one year. In the past, he's done multi-year extensions.

The first time Leno left late-night, Letterman ascended to the throne. Not this time. Since Fallon began at "Tonight," his show has averaged 5.2 million viewers, while Letterman has averaged 2.7 million and Kimmel 2.65 million, the Nielsen company said. Last year Letterman averaged 2.9 million and Kimmel 2.5 million, so the direction was clear.

Much of late-night now is about making an impression in social media, or in highlight clips that people can watch on their devices and spread around the next day. Fallon and Kimmel have excelled in spreading their comedy beyond their time slots; Letterman has barely bothered.

Late-night television is a far different world than when Letterman and Leno began their competition. There are more entertainment shows to choose from, with personalities like O'Brien, Arsenio Hall, Jon Stewart, Stephen Colbert, and Chelsea Handler working every night.

CBS will first have to decide whether or not to continue with an entertainment program in that time slot. It's not the money-maker it once was, but chances are the network will continue in that direction.

The first in-house candidate would be Craig Ferguson of "The Late Late Show," which currently airs at 12:35 a.m. on CBS and is produced by Letterman. But Ferguson's star has dimmed, his show quickly passed by in the ratings by Seth Meyers on NBC, and he is considered an unlikely choice.

A month ago, Kimmel was asked by TV Guide magazine whether he would be interested in succeeding Letterman, and he didn't shoot down the idea.

"I'd definitely consider it," Kimmel said. "I am loyal to ABC and grateful to them for giving me a shot. I was a guy from 'The Man Show' when they put me on. I'm not looking to flee. But just getting a call from Dave would be big for me. So it's definitely something I would listen to.'"

Could Leno come back? He's not the retiring type, but he would hardly be considered a play for the next generation.

Handler has let it be known that she's ready to end her show on the E! network. A broadcast network gig again would be a step up for O'Brien. Colbert and Stewart both are considered major talents and CBS would be much more high-profile than Comedy Central. John Oliver is about to start a new late-night show on HBO.

The question is whether those personalities would have too narrow an appeal for CBS, which is the broadest of the broadcast networks and would likely be looking for someone with wide appeal. Remember, many in TV considered O'Brien's "Tonight" show tenure a failure because his appeal was too limited.

Another possibility could be Drew Carey, a hit on CBS daytime with "The Price is Right" who recently traded jobs for a day with Ferguson.

Another possible decision for CBS is whether to move the New York-based "Late Show" to Los Angeles, now that "Tonight" has moved back to New York after decades on the West Coast. Los Angeles Mayor Eric Garcetti wasted no time on Thursday in firing off an open letter to CBS boss Leslie Moonves, encouraging him to relocate "Late Show" to LA.

Wherever they're located, Letterman's replacement will face a real challenge with Fallon and Kimmel, who seem to have set up a bicoastal rivalry for years to come. Fallon is now king of the East Coast, and Kimmel currently rules out West.

"David Letterman announces that he will retire next year," comic Albert Brooks tweeted on Thursday. "CBS frantically looking for someone named Jimmy."

Besides the Top Ten lists, the monologue and occasional wild visit from Bill Murray, one facet of Letterman's show that will be most sorely missed is his ability to do sharp, even hard-hitting interviews with people in the news. His first show after the Sept. 11 terrorist attacks was memorable for his reaction. It's hard to think of anyone who has the gravitas or ability to fill the role that Letterman fills.

CBS Corp. and Moonves will have time to think of that over the next year, much of which will be spent celebrating Letterman's legacy.