Student loans are gobbling up a growing share of household debt. Borrowing for education accounted for 10.2 percent of that debt at the end of 2015, about three times as much as in 2005. That makes student loans second only to mortgage debt on family balance sheets, according to data from the Federal Reserve Bank of New York. It adds up to a massive $1.3 trillion in student loans outstanding, 11.5 percent of which is overdue by 90 days or more.

This tower of debt has loomed up in the U.S. presidential race. Bernie Sanders has promised to make public college free, as Germany and Scandinavian nations have, and Hillary Clinton has her "New College Compact." Donald Trump hasn't issued a detailed plan but has criticized the federal government for profiting on student loans.

It's not just young borrowers who are loaded up with student debt. Between 2004 and 2014, the number of borrowers over 40 rose almost twice as quickly as the number of younger borrowers, according to the New York Fed. Over that 10-year period, balances held by borrowers over 60 increased 850 percent. Last year, according to financial website Edvisors, college graduates left campus with an average of $35,051 in student loans, the highest level ever.