CBS' (CBS) advertising revenue declined in the fourth quarter and in 2011 despite the network's being home to some of the best and most popular shows on television, including "Big Bang Theory," "Two Broke Girls" and "Person of Interest." How sad is that?

The Tiffany network cited the usual suspects, including lower political advertising sales and lost revenue from the NBA lockout. Advertising revenue fell to $2.51 billion from $2.62 billion in the fourth quarter and to $9 billion from $9.15 billion last year. Fourth-quarter revenue figures missed Wall Street estimates. Net income, however, rose 31% as rising license revenue and cost cutting offset the slowdown in advertising.

CBS needs to address the fact that the network TV audience is shrinking and getting older.

According to data from Nielsen, the median age of the CBS audience rose between 2006 and 2011 from 53 to 56, the oldest of any broadcast network. During the same time, Walt Disney's (DIS) ABC audience age rose from 47.4 to 52.3. Comcast's (CMCSA) NBC audience age rose to 50.1 from 48.5, while News Corp's (NWS) Fox audience rose to 45.4 from 41.5.

BTIG analyst Richard Greenfield has argued that fewer people will watch traditional television in 2012 as people spend more time playing social games. Though Greenfield's call may have been early, he is right that people's media habits are changing.

The broadcast networks are not going to wither away anytime soon. Indeed, the media companies are arguing, rightly, that older audiences are not as "bad" as advertisers believed. For one thing, they have more disposable income than younger people. The network audience is an ideal audience for advertisers that target older consumers such as financial services firms and makers of luxury cars.

Shares CBS are up more than 32% over the past 52 weeks. The average one-year price target on the stock is $32.43, about 10% above where it trades now. Investors should wait to see whether advertising sales will rebound before buying CBS shares.

Jonathan Berr does not hold any shares mentioned here.