Ask millennials how they're doing in saving for retirement and they'll tell you they're doing just fine. On the surface, that seems to be true. A new study by Principal Financial Group asked millennials (born between 1979 and 1992) about their retirement planning. Nearly 68 percent had started some sort of retirement savings before or at age 25. Six out of 10 millennials say they expect to be in better financial shape than their parents.

Yet according to the same 2015 Millennial Research Study, only 30 percent of those surveyed are saving 10 percent or more through their employer-sponsored retirement plan. Not that they don't want to—74 percent say they should be saving at least that much. However, other financial constraints are getting in the way.

The biggest demand on a millennial's income is paying the rent or mortgage, amounting to 65 percent of their monthly expenditures. Plus, millennials are working off student loan debt—over two-thirds (68 percent) of millennials surveyed have student loans. According to the National Center for Education Statistics, the graduating class of 2014 averages $33,000 in loans.