Conan O’Brien made a triumphant exit from NBC Friday night, accompanied by one of the most impressive outpourings of support by younger viewers that any late-night host had ever seen  and Mr. O’Brien was effusive in his thanks for their backing.

But he might have said: Where were you when I needed you?

In the tumult that surrounded NBC’s late-night shake-up last week, one thing was certain: If even a small fraction of the additional younger viewers who flocked to Mr. O’Brien’s show last week had turned up regularly in his earlier ratings results, he would almost surely still be hosting “The Tonight Show.”

Instead, for most of this past fall, Mr. O’Brien struggled to command the young viewers he needed to counter a falloff in overall audience numbers.

While he began to drop behind his chief late-night competitor, David Letterman on CBS, among total viewers, it was more alarming to NBC that Mr. O’Brien was not consistently beating Mr. Letterman in several important advertising-sales demographic groups  viewers 18 to 49 and 25 to 54. (He did beat Mr. Letterman virtually all the time in the 18-to-34 group.)