NEW YORK (TheStreet) -- Millennials are advertisers' Xanadu -- a promised land filled with dollar signs and the prospect of lifetime brand loyalty. Hooking the 18- to 35-year-old market has become a marketer's obsession

Putting aside millennials' worst reputed habits (entitlement, narcissism, laziness), younger consumer's spending is nothing short of dazzling: millennials spent $367.4 billion on non-essential discretionary items last year, accounting for 21% of total U.S. spending, according to research house Experian. And that comes as Millennials earned 30% less income in 2013 than the national average.

At this year's Advertising Week, the four-day industry conference that concludes Thursday in New York, advertisers and online publishers agree that their focus is the elusive roughly 77-million members of Gen-Y market, and that the best way to reach them is clearly mobile.

"The consumers' shift to mobile has happened and Millennials are basically mobile-first," said Susan Panico, Pandora's (P) head of marketing, during a Q&A session.

Among the youngest bracket of the demographic, 18-year-olds to 24-year-olds, smartphone use totals 5.2 hours per day, nearly two hours more than the average user, according to eMarketer. And smartphone usage is only increasing. Currently, nearly 86% of 18 to 35-year-olds own a smartphone, up from 78% a year earlier and well above the U.S. average of 71%, according to Nielsen, the media research company.

But for content producers to capture youth's attention, it's more than a mentality of "build it and they shall come." Millennials's consumer habits are fundamental different than preceeding generations, says Jason Kint, CEO of publishers' association Digital Content Next.

"They certainly seem to snack more in bits," Kint said in an interview. "You're trying to get the information to the consumer as quickly as possible in a consumable way. That's why Twitter (TWTR) - Get Report has been so powerful."

The key, too, is to rethink the newssite as a destination while weaving the brand into Web locations that Millennial already visit (read: social media). "You have to be everywhere they are rather than creating these bright shining beacons and hoping they come to you," Laurel Pinson, editor-in-chief of popular Millennial site StyleCaster.

Take AOL's (AOL) Huffington Post as example, a site built from the ground-up as a digital brand. The site topped the list of English-language publishers mid-year with the most content on Facebook (FB) - Get Report , six times the number of articles Buzzfeed had shared, and racking up a total 51 million impressions on the social network (Facebook 'likes' and comments, for example). According to its parent company, 53 million Millennials visit at least one AOL property, including Huffington or tech site TechCrunch, a month.

"Sharing is a huge part of [Millennials'] fabric," says Kint. "It's part of their identity -- what information they know, what brands they use."

Surprisingly, examples of public media companies appealing to the market in an authentic way are scant. Pinson, when asked, pointed to privately-held Hearst's Cosmopolitan and Conde Nast's Style, but for public companies, "I think it says a lot that nothing comes to mind."

It's a dire oversight for media companies to overlook the demographic with advertisers willing to splash huge amounts of cash to capture the market. Globally, mobile advertising revenue more than doubled in 2013 to top out at $16 billion, according to Magna Global. eMarketer forecasts that mobile advertising spending in the U.S. will more than triple to $58.3 billion by 2018. In fact, this year the medium should outpace newspapers, magazines and radio, as brands increase mobile expenditure by 83%.

The challenge, though, is for publishers to think on their feet and move quickly because the next generation of consumers are already here. "I don't think we understand Millennials enough yet and now we've got our first generation of native digitals," said Kint.

"My daughter walks up to the TV at a Best Buy and was trying to swipe and she was completely dumbfounded, perplexed as to why that wouldn't happen," added Panico. "You have this next generation that's so connected."

And that presents a billion-dollar opportunity for advertisers, brands and publishers to connect.

-- Written by Keris Alison Lahiff in New York