There are a dizzying number of age groups in this quotation:

"[I]t 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.)" (from the NYTimes)