Here's a remarkable graphic from the Wall Street Journal that shows how the audiences for various late night shows have changed over the last five years when it comes to viewers 18-34.
Notice how NBC turned things around by dumping Jay Leno and installing Jimmy Fallon, while the numbers for David Letterman have kept getting worse (to the point where he draws the fewest young viewers). It remains to be seen whether CBS can get a young-demo bump when Stephen Colbert takes over "The Late Show." Meanwhile, Comedy Central has to be concerned about whether its slides in the last year will continue once Jon Stewart is gone.
Side note: once Stewart gives up his "Daily Show" desk in a few months, Jimmy Kimmel will become the host with the longest-running show in late night. Conan O'Brien (who should have been included in this chart) has been at it longer, but with three different shows and one long interruption.