Senior citizens owe $18 billion US worth of student debt, an increase of more than 600 per cent in less than a decade, official data from the U.S. government shows.

In 2005, Americans aged 65 and older owed $2.8 billion. That figure jumped to more than $18.2 billion last year, according to new data out Wednesday from the Government Accountability Office.

In terms of the total percentage of student debt ($1.2 trillion at last count) the percentage held by seniors isn't large. But there are now 1.7 million different student loans registered to people aged 65 and up in the U.S. — a four-fold increase since 2005.

More than 80 per cent of that is their own debt, with the remaining 20 per cent of those loans originally taken out related to the education of other family members. "We do not know the extent to which the debt of older Americans is attributable to recently originated loans or loans originated many years ago during their prime educational years," the report said.

They may be a smaller piece of the overall student debt pie, but the numbers suggest they're actually more likely to default on their student loans. About 12 per cent of those with student loans aged between 25 and 49 years old are in default — meaning they have made no payments to pay down their debt in the previous nine months.

But that ratio jumps to 27 per cent for the over-65 demographic. Among those 75 years old or older, the default rate jumps to above 50 per cent.

Among households headed by those 65 and older, the estimated median student debt was about $12,000​, the GAO report said.

Unlike other forms of debt, most student loans can't be wiped out by bankruptcy proceedings, and an increasing number of American seniors are now seeing their Social Security benefits garnished to pay decades-old student debts.

About 33,000 borrowers age 65 and over had their Social Security retirement or survivor benefits offset in 2013 to repay defaulted federal student loans.

"As the amount of student loan debt held by Americans age 65 and older increases, the prospect of default implies greater financial risk for those at or near retirement," the GAO said.