Nearly four out of 10 of them have a tattoo. Almost a quarter sport a piercing someplace other than an earlobe. Most sleep with their cellphone next to their bed. They're more ethnically and racially diverse than their elders. They're optimistic and confident.

One cable network chief called them "the new Greatest Generation," and they are—to TV programmers and advertisers. They're Millennials, America's biggest generation, larger than either the baby boomers or Gen X.

Millennials, born after 1980 and currently running the age gamut from teenagers to those in their early 30s, are a force to be reckoned with in the media world. Cable is aggressively courting this young generation, which constitutes about 92 million people 12 to 34 years old, or about 68 million people in the coveted 18-to-34 demographic, according to Nielsen.

Two networks specifically targeting Millennials will launch later this year. pivot, owned by documentary film producer Participant Media, will debut Aug. 1. A month later, News Corp.'s FX Networks will unveil FXX, whose cornerstones will be established FX comedies such as "It's Always Sunny in Philadelphia."

ABC Family and MTV, among others, already have carved out nice slices of Millennial viewership, and say they aren't fazed that competitors are eyeing this demographic.

"It confirms the importance of this audience," MTV President Stephen Friedman says. "It's not surprising other people are jumping into this space to reach them. Given that they're bigger than boomers, think about the clout and the buying power."

pivot's and FXX's strategy is to find fresh takes on what they see as timeworn genres, such as talk, scripted drama and variety shows. Sitcoms and so-called "reality" programs that attract older generations don't cut it with Millennials. The two network start-ups also see great opportunities to grab young viewers in late night, where talk show audiences skew older.

"Dude-behind-a-desk is a little bit of a format that's looked at as your father's, your mother's format," FX Networks President John Landgraf says.

pivot and FXX also pledge to produce original programming that reflects life as Millennials know it: a world with nontraditional families and an ethnic, racial and sexual-orientation rainbow.

In addition, programmers say they have to engage multitasking Millennial viewers on multiple screens and on social media platforms. Twitter, Facebook and Instagram have become the new "digital water coolers" of this generation, according to ABC Family President Michael Riley.

"We sum it up in one really long word: television-viewing-fan-tweeting-app-using-blog-discussing-text-chatting-mobile-watching-consumers," Mr. Riley says.

Passionate fans want to discuss ABC Family's "Pretty Little Liars" and MTV's "Teen Wolf" live, in real time, with their peers and cast members, say network officials. "Pretty Little Liars," the top-rated show among female Millennials, currently holds the crown as the No. 1-tweeted series on television.

pivot and FXX will be under pressure to find similar success with social media.

Millennials still prefer to watch TV live in the traditional manner, according to Evan Shapiro, president of Participant Television and pivot, but they also want to gab about it while it's on. They don't want to be "left out of the social media conversation" by missing the finale of AMC's monster hit "The Walking Dead," for example, he says. Three-quarters of all social media posts about a TV show happen when it's airing live, Mr. Shapiro says.

"I like to say that must-see TV has become hashtag TV," he says.

Mr. Shapiro can't help but cheerlead for the Millennials, who he describes as "the new Greatest Generation." His characterizations of Millennials echo the results of a Pew Research study, which documented their upbeat, liberal and open-to-change philosophy—as well as their penchant for tattoos, piercings and cellphones.

"They're actually underserved as a group on television, considering that right now the Millennial generation is 60 percent of the 18-to-49 demographic," Mr. Shapiro says. "They are the most important demographic on a moving-forward basis in any kind of media conversation."

pivot, which says its goal is to entertain as well as to inspire social change, already has green-lighted six series. Mr. Shapiro says several of them will upend their genres in an effort to appeal to a young audience. The lineup includes "TakePart Live," a late-night daily talk show; "HitRECord on TV!" a reinvention of the variety show featuring actor Joseph Gordon-Levitt; "Raising McCain," a "docu-talk series" featuring Millennial Republican Meghan McCain; and "Jersey Strong," a reality show set in Newark, N.J.

"Jersey Strong," for example, is an "authentic, documentary-style" program about two former gang members and a defense lawyer, who is a gay woman raising two children with her partner. It's not staged or "manufactured for the camera," Mr. Shapiro says.

"These are characters that are not being represented on television," he says. "America isn't a bunch of middle-aged women shopping for diamonds and furs and throwing wine at each other ... America is Newark, N.J., and Baltimore, Md., and Detroit, Mich., and Oakland, Calif., where there are the haves and the have-nots."

To appeal to the large number of Hispanic Millennials, the network will be partnering with Univision to co-produce 10 documentaries that will run in English on pivot and in Spanish on the Hispanic broadcast network.

pivot's biggest gambit is to offer the network as what's billed a "first-of-a-kind" broadband-only subscription service.

Despite some criticism in the industry, Mr. Shapiro defends the subscription fee, saying, "The next generation of media consumers—potentially our best customers—consume their media in a whole new way. We believe our industry must change or risk losing this generation from the ecosystem forever."

When FXX debuts Sept. 2, Mr. Landgraf says it will have an advantage over its rivals: the brand-name association with FX and its push-the-envelope, award-winning portfolio of original programming.

With its new channel, FX Networks is borrowing a tactic from children's networks, which have stand-alone services and programming that target segments of their audiences, such as preschoolers, tweens and teens. Similarly, while FX will continue serving the broader 18-to-49-year-old audience, FXX will more narrowly target viewers 18 to 34, and movie channel FXM will try to lure the 25-to-54-year-old audience.

Millennials don't want their TV shows to be too polished; they "like rawness and spontaneity," and FX has a successful track record in that arena, Mr. Landgraf says. This new generation also isn't wed to traditional values or traditional TV: It grew up on cable.

"They don't have the same feelings about nuclear families and monogamy and gay marriage and diversity or racial issues," Mr. Landgraf says. "They are not offended, by and large, by the kind of things that older viewers find offensive. The safety and predictability of broadcast television doesn't especially appeal to them. And they don't have the same allegiance to it."

FXX's schedule will initially include comedies moved over from FX, including "It's Always Sunny in Philadelphia," "The League," "Legit" and its late-night block, "Fully Baked." Additional original comedies, as well as dramas, will be added to FXX over time.

ABC Family has reaped nearly a decade of consecutive ratings growth with programming hits such as "Pretty Little Liars" that appeal to female Millennials while drawing older women as well. The network is a ratings powerhouse in the female 18-to-34 and 12-to-34 demographics.

Millennials want "real and relatable characters," and storytelling that involves "family and friendship," Mr. Riley says. But like Mr. Landgraf, he says that younger viewers don't expect to see traditional families.

Reflecting that, this summer ABC Family will debut "The Fosters," a drama from Jennifer Lopez's production company that's about a multiethnic family of foster and biological children being raised by two mothers, a lesbian couple.

The network has strived to form a deep engagement with its viewers, using social media as a driver, Mr. Riley says. Its biggest success in that arena has been with "Pretty Little Liars," whose winter season finale drew 1 million tweets.

Young people have always been MTV's bread and butter, but the network had to make a "seismic transition" in 2008 to cater to the up-and-coming Millennial generation, Mr. Friedman says.

"We had to say good-bye to Generation X," he says. "They were beginning to age out and this massive group was coming in. ... We had to do what MTV has had to do from its beginning—shed our skin and pretty much reinvent everything."

MTV had 1 million Facebook fans in 2008, and now it has more than 150 million, Mr. Friedman says. The programming starts on TV, but the conversations about it continue on social media, even during long periods when the programs are on hiatus, he says. "We were keeping the vibrancy of the fan interest alive on all these other screens."