Millennials are often blamed for making some questionable financial decisions, like "Fomo spending," wasting too much money on avocado toast and renting instead of buying a home.

But Gen Z is already making money mistakes too.

"The biggest mistake teens make is the assumption that debt is free," Kevin O'Leary, financial expert and star of ABC's "Shark Tank" tells CNBC Make It.

"People use credit cards in a way that's really extraordinary. They assume that it doesn't cost anything to put anything on credit."

The truth is, however, most credit cards carry hefty interest rates if you don't pay off the balance every month. In October, CNBC reported that the average credit card interest rate spiked to 17.01 percent from 16.15 percent a year earlier and 15.22 percent two years ago.