The AARP has two basic issues with the current student loan situation. The first is that an increasing number of elder Americans have defaulted on their loans. As a result, their Social Security benefits are being garnished. The organization is trying to get a law passed to end the garnishment of Social Security benefits.
More generally, the AARP is worried that the increasing amount of debt graduates are carrying is making it difficult for them to save for retirement. Americans generally do not save enough for their retirements. If the student loan situation gets worse in a few decades, elder Americans could be even more unprepared to retire.
Reference: Politico (June 7, 2018) "Why the AARP is worried about student loans."