Late Night Ratings: The NYT sums it up in one sentence

"In Late-Night Battle, It’s All About Young and Old." Even when Conan is in reruns, younger people are still watching "Tonight," while the older demographic flocks to "Late Show with David Letterman."

Even though Dave had almost a million more viewers than Conan, O'Brien still beat him in "adults between 18 and 34 (598,000 to 336,000) and won easily in the category sought most avidly by many advertisers, viewers between 18 and 49, 1.280 million to 1.068 million."

In total viewers, the "Tonight" repeats were watched by 2.468 million viewers, while 3.407 million tuned in to see the fresh "Late Show"s.