NEW YORK (MainStreet) — Millennials will make up more than one of three adult Americans by 2020 and be about 75% of the workforce by 2025, the Brookings Institute says, but these soon-to-be working adults aren't like today's adults.

For one thing, 63% of millennials don't even own a credit card, according to Bankrate.com.

That's surprising, given how pervasive credit cards are today, and has deep ramifications for younger Americans who could really benefit from generating good credit from solid card management skills, says Bankrate.com analyst Jeanine Skowronski.

Rejecting credit cards could cause financial troubles for millennials because a strong credit score — often obtained through the responsible use of credit cards — is the foundation for a stress-free financial life, she says.

"Millennials may think they're staying out of financial trouble by forgoing credit cards, but they're actually doing a disservice to themselves and their credit scores," Skowronski says. "The responsible use of credit cards is one of the easiest ways to build a strong credit score, which is essential for qualifying for insurance policies, auto and mortgage loans and sometimes even a job."

Jeannie Horner, an associate at New York City public relations firm Alison+Partners,  who works out of the firm's San Diego office, made it to 29 before getting a card.

"I do have a credit card, and in fact just got it recently," Horner says. "But I actually haven't used it yet, since I only opened it for my upcoming international trips."

Her first-ever credit card purchase will be to book two international airline tickets to Indonesia and two weeks' worth of hotels, she says, getting cash back rewards and added security.

Horner says she doesn't plan to use the credit card for everyday purchases.

"There are some reasons I haven't used credit cards in the past and still don't plan to use them regularly," she says. "I only spend money that I have. That's how I was raised."

That puts her in good company. A TD Bank survey shows that many millennials are cautious financially, and 47% "describe their financial personality as being cautious when it comes to overall personal finance habits."

"I saw way too many friends get upside-down in credit card debt in their early or mid-20s, and they're still trying to dig themselves out, and I've always figured that credit cards come with a bunch of hidden fees and other unclear 'rules' and policies, since banks in general seem to do that," Horner says.

While millennials such as Horner shy away from credit cards, they are the least likely demographic group to pay their bills on time when they do use credit cards. Bankrate says only 40% of millennials pay their credit card balance in full and on time every month - significantly below the 53% of Americans over the age of 30 who pay their credit card balance in full every month.

This may have a lot to do with the "personal" economy millennials experience, including their own debt issues.

"Many millennials are already battling with student loans, which likely makes them even more wary of the potential for debt," Skowronski said.

With so many younger Americans struggling with loan debt, they also might lack the good credit needed to qualify for low-interest rate credit cards.

Once a younger consumer pays off student loan debt, better credit and access to credit cards should follow.

Let's hope so. With millennials making one-third of the population in six years, the economy will increasingly depend on them to spend money and keep the economy rolling.

By Brian O'Connell for MainStreet

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