The proportion of U.S. student loan balances that are in delinquency — that is, unpaid for 90 days or more — surpassed that of credit-card balances in the third quarter for the first time, according to the Federal Reserve Bank of New York.

Of the $956 billion in student-loan debt outstanding as of September, 11 percent was delinquent — up from less than 9 percent in the second quarter, and higher than the 10.5 percent of credit-card debt, which was delinquent in the third quarter. By comparison, delinquency rates on mortgages, home-equity lines of credit and auto loans stood at 5.9 percent, 4.9 percent, and 4.3 percent respectively as of September. (Read More: Debt-Free College? Yes, It Exists)

With the release of these figures, student-loan debt officially takes the crown from credit-card debt as that most prone to delinquency in the U.S. Since the NY Fed's data began in 2003, the share of student debt which is delinquent has nearly doubled, from a starting level of 6.13 percent, while credit-card delinquency has steadily drifted lower since peaking at 13.74 percent in mid-2010 in the wake of the financial crisis.