The biggest TV drama among millennials is playing off screen.

So far this season, younger viewers, the most important audience for advertisers, have ditched their TV sets at more than double the rate of previous years, new Nielsen figures show.

Traditional TV usage — which has been falling among viewers ages 18 to 34 at around 4 percent a year since 2012 — tumbled 10.6 percent between September and January.

In the era of smartphones and Netflix, it’s no surprise that traditional TV is losing relevance for younger viewers. But the sudden acceleration is alarming to even the most seasoned analysts.

“The change in behavior is stunning. The use of streaming and smartphones just year-on-year is double-digit increases,” Alan Wurtzel, NBCUniversal’s audience research chief, told The Post. “I’ve never seen that kind of change in behavior.”

Brad Adgate, Horizon Media’s chief researcher and often one of the first to spot trouble, was equally surprised at the sudden drop.

“Usage is really down in the 18- to 34-year-old demographic this season,” he said.

This season’s steep slide means there are almost 20 percent fewer young adults watching their TV sets in primetime than four years ago.

In 2011, 21.7 million young adults tuned in to their TV sets. By the end of last month, that figure had fallen to 17.8 million, according to Nielsen figures.

Adding insult to injury, the median age of the TV audience hit 50 this year. That’s older than the 18- to 49-year-old audience that network executives have banked on for decades.

If the TV-as-an-anachronism trend holds, the implications for the media industry are huge, possibly causing a seismic shift in the $80 billion TV ad market.

Of course, the trend of zero TV doesn’t mean zero video. Millennials are watching online video from Netflix, Amazon Prime, HBO GO and other “streaming” sources.

Consumption of video is bigger than ever. Wurtzel’s research shows a year-over-year increase of 22 percent in subscription video viewing in 2014, and a 26 percent rise in “binge viewing.”

The problem is finding those missing viewers — or measuring them. Nielsen’s traditional ratings don’t capture the full picture, making it tough for networks to monetize viewers who watch shows on smartphones, tablets and other devices.

Network executives are trying to get around the problem. Media giant Viacom, which owns MTV, Comedy Central and other younger-skewing networks, said recently that around 30 percent of its ad deals are based on non-Nielsen data sources.

“Industrywide declines in ratings are generating debate about ways to close the gap between currently accepted ratings and actual consumption,” Viacom CEO Philipe Dauman said in a recent earnings call.

NBC hopes to juice ratings with data on viewers who are watching shows via NBC.com and online hub Hulu. To bolster its case, the network points out that an additional half-million viewers ages 18 to 49 watch the hit show “The Blacklist” on digital devices.