A retirement organization may seem like an unlikely force for reform of student debt. But AARP’s involvement underscores just how long a shadow student loans are increasingly casting over Americans’ economic lives—a shadow that stretches all the way into their retirement. Remarkably, Americans over 60 years old are the fastest-growing category of student loan borrowers, having roughly quadrupled in number between 2005 and 2015, according to the Consumer Financial Protection Bureau. Although older borrowers still account for just a sliver of the more than $1.5 trillion in total outstanding student loan debt, they’re more likely than younger borrowers to be behind on payments. Most are repaying debt they took out to help finance the education of their children or grandchildren, though some are still paying off their own tuition.

“We consider it a looming threat,” said Lori Trawinski, director of banking and finance at the AARP Public Policy Institute. A central concern, she said, is how student debt—for themselves and their children—can delay key financial milestones like home ownership and chip away at retirement savings.