Over 44 million Americans have student loans, with the average debt hovering around $33,000. And yet that's not the No. 1 source of debt for the average older millennial.

Millennials between the ages of 25 and 34 have an average of $42,000 in debt each, according to Northwestern Mutual's 2018 Planning & Progress Study. The biggest source? Credit card debt.

Credit card balances make up a full fourth of the average older millennials owe, while student debt accounted for about 16 percent, according to Northwestern Mutual. The findings are based on a survey of over 2,000 U.S. adults, including an oversampling of more than 600 millennials.

"As you grow older, your expenses increase. The additional pressures that come onto the pocketbook only grow and your disposable income shrinks in a lot of cases, even if your salary is growing," Emily Holbrook, director of planning for Northwestern Mutual, tells CNBC Make It.

The pressure to start relying on credit cards makes a lot of sense, she says. Millennials also cope with many other financial burdens that other generations haven't had, including those high student loans, as well as self-funded retirement savings and soaring home prices, to name a few.

Across the board in America, debt is on the rise — even though most people have a strong desire to pay it off. According to Northwestern Mutual's findings, the average American has about $38,000 in debt, up $1,000 from a year ago.