The number of Americans over age 60 who are paying back federal or private student loans has reached a critical mass, quadrupling to 2.8 million over the past decade, a new report finds. These older borrowers owe $23,500, on average, and two-thirds of them also have mortgages and credit card bills at a time their medical expenses are typically increasing, according to the report issued this month by the Consumer Financial Protection Bureau (CFPB). Separately, nearly 40 percent of those with federal loans have defaulted on their payments. The response of many older student loan borrowers, the CFPB said, is to “skip necessary health care needs such as prescription medicines, doctor’s visits, and dental care because they could not afford…