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The 18 to 35-year-old generation are notorious for renting homes instead of buying, spending on experiences over apparel, and dining at restaurants instead of at home.
But that's starting to change as more millennials become parents. More than 60 million millennials - about 80% of the generation - will become parents in the next decade, according to a study by marketing platform Crowdtap.
Millennials currently spend $170 billion per year and are projected to spend $200 billion annually starting in 2017 and $10 trillion in their lifetimes, according to a study by Exponential.
Millennials are becoming parents - they're just doing it later than previous generations.
This chart using Wells Fargo data shows how the average age of first marriage and childbirth is rising in the US.
Business Insider/Wells Fargo
Once people have kids, they usually trade in happy hours and vacations for houses and cars.
This chart by JPMorgan shows that millennial spending is fairly similar to older generations. As they increasingly become parents, spending on categories like home improvement are expected to rise.
JPMorgan
Shifting attitudes toward lodging and transportation
Millennials have long said in surveys that they want to own homes like their parents, as Business Insider's Bob Bryan recently reported. Even those living in their parent's basements have expressed their intent to get their own place.
KB Homes CEO Jeffrey Mezger said in a recent earnings call that first-time home buyers and young people are beginning to enter the market, driving demand to his business.
Millennials are also buying more cars.
The largest generation in the US bought 4 million cars and trucks last year, according to the Associated Press. Their share of the new car market was at a record 28%.
New winners in the fast-food industry
Their habits as parents are also influencing the restaurant and fast-food industries.
Traditional fast food chains are losing market share to up-and-coming establishments like Panera Bread as millennial customers search for healthier options.
This shift in consumer mindsets puts Starbucks in a unique position to take over, according to a 2015 report by Goldman Sachs.
"Starbucks is virtually the only large incumbent that can offer millennial parents the convenience of a (fast food chain) and food they would not feel guilty/embarrassed to feed to their kids," Goldman Sachs' analysts write.
Thomson Reuters McDonald's recently retooled its McNuggets recipe to eliminate preservatives.
Starbucks has been expanding its menu to include more food options such as sandwiches and salads. It has also added drive-thrus to many locations.
The coffee chain has also been "steadily expanding kid-friendly snack options, such as organic fruit squeezes, organic food snacks, and organic Greek yogurt," according to Goldman Sachs.
But McDonald's, Chick-fil-A, Wendy's, and others are trying to catch up by eliminating preservatives in food and offering more healthy and organic options.
Grocery stores make a change
Companies like Target, Walmart, and Kroger are making changes to appeal to millennial moms, who tend to do most of the shopping for their households.
Target's CEO has been traveling around the country visiting the homes of millennial moms to better understand how they shop.
Walmart and Kroger have been offering wider selections of organic foods, which are important to the health-conscious generation. Kroger has also been expanding its private-label products so it can offer cheaper prices.
Whole Foods is opening a new chain of stores to better appeal to millennial moms, who crave convenience and well-organized aisles.
The company's new 365 stores will be much smaller than regular stores, offer lower prices, and cater to more technology-savvy consumers.
Reuters
Traditional apparel chains struggle, athletic retailers win
"The fashion industry has undergone one of the most dramatic makeovers in recent history - no doubt influenced by the millennial consumer," said Marshal Cohen, chief industry analyst at NPD.
While US consumers spent slightly more on apparel, footwear, and accessories in 2014, those increases were driven by the new "athleisure" trend, not by traditional retailers like Macy's, NPD says.
While previous generations dressed up for work and play, millennials have a tendency to wear brands like Nike, Under Armour, and Lululemon everywhere, according to NPD.
"There is an underlying sense of rebellion that comes through in today's fashion," Cohen said.
While department and discount stores struggle, business for athletic retailers is booming.
Cohen says that traditional retailers will have to evolve to newly casual consumers.
"This is no longer a trend - it is now a lifestyle that is too comfortable, for consumers of all ages, for it to go away anytime soon," he said.
It's likely millennials will pass their fashion preferences on to their children, too.
Hayley Peterson contributed to this story.