More than six in 10 millennials (63%) do not have a credit card, according to a newBankrate.com report. Comparatively, only 35 percent of adults 30 and over have zero credit cards in their wallets.

Millennials are the least likely of any age group to pay their balances in full each month. Only 40% of millennials pay their entire balance every month, compared to 53% of adults 30 and older. Three percent of millennials admit to often missing payments completely, more than any other age group.

"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," said Jeanine Skowronski, Bankrate.com's credit card analyst. "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."

Recent polls suggest that more Americans, not just millennials, are relying less on credit cards since the Great Recession. But millennials in particular may have a greater aversion to debt since they grew up witnessing its effects on the economy.

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

One other reason millennials don't have wallets full of plastic is simply because it is harder for Americans under 21 years old to get a credit card because of new legislation such as the Credit Card Accountability, Responsibility and Disclosure Act of 2009 (the CARD Act).