The good news for Conan O'Brien is that his talk-show premiere on TBS on Monday night blew away all rivals in the ratings, including Jon Stewart and even the man who took his place at NBC, Jay Leno.

The bad news is that O'Brien's hard work is just beginning.

The 11 p.m. "Conan" rounded up 4.2 million total viewers, according to early data from the Nielsen Co. That soared straight past NBC's "Tonight Show with Jay Leno" (3.5 million) and CBS' "Late Show With David Letterman" (3.4 million), both of which start at 11:35 p.m. Perhaps of more interest to late-night connoisseurs, however, was that in head-to-head competition during his hour-long time slot, "Conan" put a serious dent in the "Comedy Central" power block of "The Daily Show with Jon Stewart" (1.3 million) and "The Colbert Report" (1 million). Last Monday, "Daily Show" delivered 1.9 million viewers.

Best news of all for TBS? The youth of the "Conan" audience, which makes it a relatively easy sell for advertisers. The median age of the O'Brien viewer was 30, which is music to a marketer's ear. By comparison, the comparable figure for "Daily Show" was 38. For "Tonight," it was a relatively ancient 59. For "Letterman," it was 53. A more stark illustration of the generational dynamics among the various talk shows cannot be found.

The bad part for O'Brien, though, is that premieres, especially of talk shows, generally tell us very little. Most talk shows with big-name hosts attached generate very high numbers at first. Sometimes, these programs fizzle out over the ensuing months as novelty and viewer curiosity wane. That is what happened with Leno's disastrous prime-time talk show for NBC. O'Brien's real performance, therefore, won't be known for weeks or months.

But for now, O'Brien's decision to exit NBC rather than accept a post-Leno downgrade to the wee hours is looking very smart.

-- Scott Collins (Twitter: @scottcollinsLAT)

Photo: Conan O'Brien performing his first monologue for TBS. Credit: Meghan Sinclair/TBS