NEW YORK — 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.


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.


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.


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 barely bothers.


CBS will first have to decide whether or not to continue with an entertainment program in that time slot.


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.


The question is whether those personalities would have too narrow an appeal for CBS, which is the broadest of the broadcast networks and would be looking for someone with wide appeal.


Another possibility could be Drew Carey, a hit on CBS daytime with “The Price is Right.”


Another possible decision for CBS is whether to move the New York-based “Late Show” to Los Angeles. Los Angeles Mayor Eric Garcetti wasted no time firing off an open letter to CBS boss Leslie Moonves, encouraging him to relocate “Late Show” to LA.


“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 ability to fill the role that Letterman fills.


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