No matter what they slap on the air this fall, television execs can bank on a nice ratings boost.

For the first time, the Nielsen ratings that are the lynchpin of the TV business will start counting viewers who watch shows via mobile apps on tablets, phones and other handheld devices.

Folding in mobile viewing is expected to add hundreds of thousands of new viewers overnight, according to several industry estimates.

Even better for networks dealing with youth-obsessed advertisers: the average age of their audience will skew younger.

Steve Hasker, Nielsen’s president of global product leadership, predicted that after years of declines, live TV ratings will “be flat or up” this season.

Hasker, speaking at a media conference on Thursday, said demand for Nielsen’s mobile ratings product has been off the charts as networks look to boost their numbers after years of live ratings erosion.

Consumers have been reading less and even visiting malls less in favor of watching online video, he added.

Nielsen’s latest “Cross Platform Report” shows that consumers spent 126 minutes a day using their phones, up from 48 minutes in 2012.

Brad Adgate, chief researcher for Horizon Media, said mobile ratings would be a boon for the online video space as well.

“The viewers are significantly younger,” Adgate said.

The median age of a primetime TV viewer is 55; it was 40 years old two decades ago.

Investors have been skittish about cable and broadcast stocks given the weak “upfront” ad sales market in the spring and fears that TV dollars are migrating to digital.

But some advertisers may be sitting on the sidelines, knowing there could be more eyeballs to buy in the future.

“Next year’s upfront will look very different,” said one media analyst.

David Poltrack, CBS’s chief research officer, said the move to measure mobile involves several positive developments for the networks.

For one, video-on-demand is growing faster than DVR playback. The networks can disable fast-forwarding on VOD platforms as opposed to ad-skipping DVRs.

There’s also dynamic ad insertion. Advertisers in the current episode of CBS’s “Big Bang Theory,” for instance, will also show up when viewers watch older episodes, enabling networks to get credited in the ratings.

Just as important for the targeted advertising model: “Nielsen will have the ability to identify within the app who the individual viewer is,” Poltrack said.

“Viewing via WiFi in the home has been around for over a year now, and people are using it more and more,” he said. “ We don’t have any good measure of that viewing.”

While many have criticized Nielsen for its slow move to measure mobile, the company was forced to wait while major clients sorted out their mobile rights issues and in some cases their entire business model, according to Hasker.

“Some didn’t know what their business models were; they weren’t ready,” he said at the Goldman Sachs Communacopia conference on Thursday. “But we were right on it and were ahead of our customers.”