Viewers have yet to begin flocking to watch David Letterman in his final season as host of the Late Show on CBS. In fact, viewership from the start of the new broadcast television season on Sept. 22 through the end of January is actually down by about 230,000.

However, Brad Adgate, senior VP, research at Horizon Media, says the network hasn’t been actively promoting Letterman’s departure, and he expects audiences will start to build as it gets closer to his May 20 finale. CBS will air repeats through the summer until Stephen Colbert takes over as host on Sept. 8.

“I think it is going to be one of the top TV events of the year and will be the largest audience on late night since Johnny Carson left the Tonight Show in May 1992,” Adgate says of Letterman’s final show. “As the current TV season winds down and as CBS publicity ramps up, more casual viewers will start tuning in. And he’s expected to have a lot of big-name guests stopping in to say good-bye, including an expected appearance by Jay Leno. I think viewership of his final show will top the audience Leno drew for his, but I don’t think it will reach Carson’s. That was just a different TV landscape with less competition and audience fragmentation.”

Leno’s finale in 2014 drew 14.6 million viewers, while Carson’s last show in 1992 pulled in 50 million viewers.

This season, Late Show with David Letterman has averaged 2.66 million viewers, down from 2.89 million last season-to-date. Letterman has lost about 100,000 viewers in each of the 18-49 and 25-54 age demos, and based on an examination of Nielsen data, those viewers were picked up by The Tonight Show Starring Jimmy Fallon.

Overall in the 11:35 p.m. time period, Fallon is leading with 3.7 million viewers, with Letterman in a virtual dead heat with Jimmy Kimmel Live, which is averaging 2.64 million viewers.

Letterman has passed Carson as the longest-running late-night TV host, and during the time he’s hosted Late Show, beginning in August 1993, the daypart’s landscape has changed dramatically. Competition from myriad late-night cable programming has steadily pulled viewers away from the broadcast entries.

During his first full year of 1993-94, Letterman averaged 7.2 million viewers, including 4.5 million viewers in the 18-49 demo. Back then, the late-night shows on broadcast were drawing more 18-49 viewers than 25-54. During that year, Late Show had a median-age audience of 39 (this season, it’s 60), and Letterman was the late-night leader with The Tonight Show averaging 5.5 million viewers with a median-age audience of 44.

This season in broadcast late night, Jimmy Fallon is not only the viewer leader by a wide margin with 3.7 million, but he also draws the most women (2.2 million), the most 18-49 viewers (1.3 million) and the most 25-54 viewers (1.6 million). While having a median-age audience of 55, the lowest of the three broadcast 11:35 p.m. late-night shows, Fallon also draws the most 55-plus viewers (1.9 million).

Kimmel has a median-age audience of 57 for his 2.64 million viewers that include 1.7 million women. He also draws 783,000 adults 18-49, 1 million adults 25-54 and 1.5 million adults 55-plus.

Letterman, with his 2.66 million viewers and a median-age of 60 draws 1.5 million women, 636,000 adults 18-49, 857,000 adults 25-54 and 1.7 million adults 55-plus.

Since Fallon took over for Leno in mid-February 2014, his comparison to last season-to-date has to be vs. Leno’s numbers rather than his own. Fallon’s 3.7 million viewer total through January is 500,000 less than Leno’s but he is drawing 200,000 more 18-49 viewers and 200,000 more 25-54 viewers than Leno did. And Fallon is also drawing 200,000 fewer viewers 55-plus than Leno.

Kimmel, who had his show moved from 12:35 a.m. to 11:35 p.m. in January 2013, has seen his median-age audience jump 3 years since last year. His viewership is up by a little more than 100,000 season-to-date but he’s lost viewers in the younger demos and picked them up in 55-plus.

One interesting piece of Nielsen data offers that while previous ABC 11:35 p.m. occupant Nightline had a median age of 58 (compared to Kimmel’s current 57) during the 2012-13 fourth quarter, the news series averaged 3.4 million viewers, almost 800,000 a night more than Kimmel is averaging this season. And despite being perceived as an older-skewing show, Nightline drew more 18-49 and 25-54 viewers than Letterman in the time period, and was tied with Leno, drawing 1.3 million 25-54 viewers.

The problem for ABC was that there was a perception among advertisers that mostly older people watch new shows, making it harder to compete with the other entertainment talk shows for ad dollars.

Among the three “Late, Late” shows, the viewership battle is a lot tighter. Nightline averages 1.6 million viewers compared to The Late Show with Seth Meyers, which averages 1.5 million, with The Late LateShow on CBS at 1.4 million viewers.

Late Late Show host Craig Ferguson departed in mid-December and CBS has been using weekly guest hosts to fill the gap until James Corden takes over as host on March 23. The guest hosts haven’t helped bolster viewership, which is averaging the same 1.4 million viewers it did during this period last season. In fact, the median-age audience has risen by two years to 57.

Meyers has lost a chunk of the audience Jimmy Fallon had before he left to take over Tonight. Last season, The Late Show with Jimmy Fallon was averaging 1.87 million viewers, while Meyers this season is averaging 1.5 million during the same period. Myers has lost about 150,000 viewers in each of the 18-49 and 25-54 demos.

Adgate believes that Colbert replacing Letterman beginning in September is not going to knock Fallon from the top of the late-night ratings perch.

“Unless something totally extraordinary happens, Fallon will still be the leader at this time next season,” Adgate says. “But each host will have their moments.”

Adgate points out that the shift from Letterman to Colbert will save the network lots of money even if the ratings don’t soar through the roof. “Currently, Letterman’s production company produces the show, but CBS will take over ownership of the new show,” Adgate says. “And, of course, they will be paying Colbert a lot less money to host the show than they paid Letterman.”

Another area CBS will benefit is in the battle for second-screen viewership, Adgate says. Both Fallon and Kimmel have brought lots of added exposure to both of their shows and networks via skits they later streamed on YouTube and other social media sites. Letterman has not been as online savvy.

“Colbert has a big online presence and will make the show more of a competitor in the social media arena,” Adgate says. “Colbert also had a median age for his Comedy Central show in the mid-40s so he could help reduce the CBS Late Show median age which is now the highest among the three shows.”