Retirement is a milestone that millions of workers look forward to. Some seniors, however, don't wind up as enamored with it as expected. An estimated 39% of Americans age 65 and older who are currently employed were actually previously retired, according to a 2017 Rand Corp study, and the reason doesn't just boil down to money.Sure there are plenty of seniors who end up going back to work after attempting to retire due to financial concerns or constraints.