The total debt shouldered by Americans has hit another record high, rising to $13.5 trillion in the last quarter, while an unusual jump in student-loan delinquencies could provide another signal that the nation’s economic expansion is growing old.

Flows of student debt into serious delinquency — of 90 or more days — rose to 9.1 percent in the third quarter from 8.6 percent in the previous quarter, according to data from the Federal Reserve Bank of New York.

It was a reversal after a period of improvement for student debt, which totaled $1.4 trillion. Such delinquency flows have been rising on auto debt since 2012 and on credit card debt since last year, which could raise a red flag for economists.