In China, more than in most other places, there's a massive gap between millennials and their parents. Unlike previous generations in China, the so-called post-90s generation, or those born in the 1990s, grew up during boom years of double-digit growth.

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They have fewer memories of hardship than their parents, and high hopes for the future. Western brands were scarce when their parents were young; now people in remote areas can use their mobile phones to order foreign brands off the internet.

And since they grew up under the one-child policy, many enjoyed the undivided attention of both parents plus two sets of grandparents. For marketers, appealing to post-90s youth is a major fixation – after all, there are 135 million of them.

For China's youth, "their purpose in life is different than the former generation, they really want to make this moment interesting and happy rather than living for the future and for others – I think that's a critical values shift," said Ellen Hou, McCann Worldgroup Shanghai's group managing director and chief strategy officer. "The implication for brands is, how to make the brand be alive in this moment?"

Ms. Hou became one of the first generation of planners born in mainland China to work at an international agency when she joined TBWA in 1998. Today, working with McCann Shanghai's clients including Coca-Cola, L'Oreal and Wyeth, she's constantly thinking about what makes millennials tick. Ms. Hou, honored as one of Ad Age's 2015 Women to Watch China for turning insights into strategies that connect with people's lives, shared what works in youth marketing, and what no longer applies.

Keep it relaxed and lighthearted

Chinese drinking culture used to focus on building "guanxi," or connections, especially for business. "Before it was all about serious relationships, drinking as a ritual, and everyone had to drink according to the rules," said Ms. Hou. "Drinking alcohol was about closing a deal, now it's about having fun, taking a break, sharing freely and making new friends. There's a trend among the younger generation, with relationships becoming more lighthearted and casual. People are seeking authenticity in friendships, it's not always with some other purpose." Brands can keep things "qing song," or nice and easy, she said. McCann kept that insight in mind to make a new brand statement for China's Rio, the country's No. 1 bottled cocktail, which is a booming category in China. It's "Rio: Let's Make Friends."

If you nod to tradition, make it playful

For the Lunar New Year, Coca-Cola revived two seasonal mascots originally launched in 2001. Inspired by traditional Chinese clay doll folk art, the mascots are two chubby kids named A Fu and A Jiao. McCann and Coke modernized the styling and put them on packaging; people could scan the package to get animated stickers to send as holiday greetings on mobile app WeChat. "This goes back to the idea of having fun and taking it easy – you can do that with traditional Chinese culture too," Ms. Hou said. The campaign helped increase brand Coca-Cola's volume 9% despite slowing economic conditions, CEO Muhtar Kent said during the first-quarter earnings call.

Reassess what resonates

L'Oreal's Maybelline, which traditionally associated its brand with New York, shifted its messaging slightly. For Chinese youth, the focus on New York was "too distant and too vague," Ms. Hou said. "So we took the key essence of New York," she said, the idea of excitement, and things happening in the moment. The updated messaging is about "being beautiful now."

The e-commerce boom changed the game

Fashion companies are among the brands that should rethink the purpose of their brick-and-mortar stores, given how quickly China has embraced e-commerce, overtaking the U.S. to be the No. 1 market globally. McCann is helping fast fashion chain C&A on product selection, communication and store design, and it sent anthropologists to several cities to talk to young people about their fashion and shopping habits.

"The current store is more like a premium supermarket – you see tons of products," Ms. Hou said. "The new direction is more about enjoyment. The store should be a collection of 'moments' or experiences rather than a collection of products – product does not matter as much to people because you can buy lots of clothes on Taobao," the Alibaba online marketplace.

Swim in pop culture

"Before when you were working on a youth brand you'd normally do so-called 'advertising' to broadcast the message, 'we are a cool and young brand,'" Ms. Hou said. Now it's quite effective to place the brand in movies or soaps, often South Korean ones, that are popular in China. Rio, the bottled cocktail, bought product placements in seven youth-oriented shows. "Young brands need to market themselves as pop culture rather than a product or brand, and that's a big shift," she said.

The Women to Watch China questionnaire:

Home city: Shanghai

Hobbies: Painting; analyzing gossip as a form of anthropology.

First job: Sales assistant selling glass panels and windows during Shanghai's building boom.

If you could have dinner with anyone living or dead, who would it be? Shakespeare

If I had it to do all over again, I'd…. Be an anthropologist or write dramatic scripts