Someone forgot to tell American men the fall TV season has started.

As the fall season has started, the number of young men turning on their TVs dropped by a stunning 22 percent compared to last year, Nielsen ratings show.

Despite tens of millions of dollars in ad campaigns and big draws such as NFL, Fox’s young music-themed drama “Empire” and NBC’s “The Voice,” roughly 640,000 fewer men between 18 and 24 watched any show on their TVs.

Last year, 3 million men flipped on their sets between Sept. 22-24. This season, that number fell to 2.36 million during Sept. 21-23, according to Nielsen data. 

“That the [young men] figure is down to that amount is very interesting and speaks to the point there are other activities besides TV,” Horizon Media’s senior vice president of research, Brad Adgate, told The Post. 

While young men are traditionally the toughest demographic to attract, the severe audience drop-off was also evident in the number of overall people ages 18 to 49. There, the decline was 8.4 percent, to 38.2 million over the first three days — and that is just as troubling for the networks.

There are several explanations for the shocking decline, including, of course, youngsters watching video via subscription services and YouTube on their mobile devices.

The networks have been pushing Nielsen for years to speed up its measurement of people watching video outside of the traditional TV screen.

To be sure, the new season is just three days old, and the troubling trend could soon reverse — although few are predicting such a turnaround.

Several companies, including 21st Century Fox, Time Warner and CBS, are believed to have had serious conversations with Nielsen about their concerns. 

The TV business is keen to get paid for viewing beyond the initial window and traditional screen.

Currently, advertisers pay for catch-up viewing through the first three days; TV networks want to write all their deals at the very least on viewing during a full week and also for streams that happen up to 30 days beyond the initial broadcast. 

“It could be that the young men are on other devices,” Adgate said. “It could be they’re watching more OnDemand. It’s growing, and the competition online is so much keener.”

H/T NY Post