One of the hottest topics in the television industry these days is why the broadcast networks are shedding younger viewers at an alarming rate — and what it means for the business.

"There are lots of theories about why broadcasters are losing adult viewers under 50: They’re spreading out to cable, or watching shows online. Maybe network shows just aren’t what they used to be," TheWrap.com reports.

But CBS, which the report notes has the oldest median viewer age at 56.1 years, has its own view, which the story sums up as: "Networks are winning fewer viewers 18-to-49 because there are fewer people 18-49. Not only that, the network says, but those who remain don’t carry the weight they once did. They tend to skew younger, and to increasingly — gasp — live at home, even up to the age of 34."

The piece quotes CBS Corp. Chief Research Officer David F. Poltrack, one of the most respected researchers in the business, summing up younger viewers with a bit of understatement: "This makes them of limited interest to a substantial number of advertisers."

But Poltrack adds that CBS understands "the whole youth culture thing." The network last season won the ratings battle in the key 18-49 demo for the first time since 1991-92, the report notes.

"What’s become of younger adult viewers is one of the biggest questions at the Television Critics Association summer press tour," the report notes. "So CBS gave Poltrack an entire panel Monday to make the case that 18-to-49-year-old viewers don’t matter as much as they once did, and that networks and advertisers should broaden their focus beyond the one now considered most desirable."

Poltrack told the website: "This 18 to 49 demographic is a smaller percentage of the overall population in numbers. So why would anybody sell — this is what we tell advertisers — why would you want to continue to sell and focus your selling on a shrinking part of the total population?"

Other insiders offer a different spin. NBC Entertainment Chairman Bob Greenblatt, speaking over the weekend at TCA, provided a more traditional perspective: "The decline year-to-year in broadcast television from 5 to 7 percent has been happening for the last two decades. So it isn’t just a recent phenomenon, and it’s been happening partly because cable is just taking viewers away."

"The CW, meanwhile, has blamed its recent ratings woes in part on its core audience, people 18 to 34, increasingly watching TV online. CBS co-owns the network with Warner Bros. Entertainment," the report notes.

Another CBS executive, President Leslie Moonves, made the point at TCA that the Internet can be a valuable partner for the networks. "He noted that CBS’s ‘Under the Dome’ earned roughly 13.5 million viewers initially, but ticked up over 20 million including DVR and streaming audiences," the piece reports.

Said Moonves: "Since I’ve been in the network television business, which is over 30 years, people have been saying, ‘Oh, the model is dead.’ The model’s never been dead. It’s just evolving. It’s changing."

Please click on the link near the top of this story to read the full report.