Student debt isn't just a problem for Millennials. It's increasingly common — though still very rare — among the elderly, who have $18.2 billion in student debt, according to a new report from the Government Accountability Office:

The total amount of student debt held by people over 65 is still very small compared to the $1 trillion in outstanding debt. But it's grown rapidly since 2005. And the report says that most of these elderly debtors borrowed for their own education, not their children's or grandchildren's.

Many of these borrowers are struggling to pay the loans back: more than half of loans held by people over 75 are in default, meaning they haven't made a payment in at least 270 days.

When a federal student loan is in default, the government can keep portions of Social Security checks to pay it back. In 2013, that happened to 4,900 people over 75 and 28,200 between 65 and 74.