While the portion of a broadcaster's audience that falls within the advertiser target adults 18-49 demo isn't particularly significant* except as a point of discussion, discussing ratings is what we do!
So, which broadcaster has the largest portion of their primetime audience in the advertiser targeted adults 18-49 demo?
No particular surprises here, the CW has by far the largest share of its primetime audience in the 18-49 demo at 61%. Fox is second with a somewhat larger portion of its audience in the 18-49 demo (48%) than NBC (43%) and ABC (41%), and CBS, with the oldest audience of the broadcast networks, has just 36% of its audience in the adults 18-49 demo.
Fox's PR often talks about how they're growing their young audience into a long term watching asset for them. I'd love to see numbers that back that up. Logically, it seems just as likely to me that as Fox's audience ages, they'll just start watching CBS!
* What is significant to the business of the broadcast networks is the absolute audience size in the adults 18-49 demo, (although the CW targets adults 18-34) that correlates to advertising income.
Chart includes the first 28 weeks of the 2009-2010 broadcast season through April 4, 2010.