Millennials have more debt and less income than Gen X or baby boomers on average. But those aren't the only financial constraints some face. New data show 1 in 5 supported an aging parent over the last year, spending an average of $18,250 a year on medical bills and other expenses — more than any other generation, according to TD Ameritrade's recent 2015 Financial Support survey.

That single expense puts a serious strain on their ability to pay off student loans and save for retirement. (Consequently, baby boomer parents have also delayed their retirement plans to help their children financially)

"It's a lot harder to keep your head above water if you have to support family members and still try to tackle your debt," said Matt Sadowsky, TD Ameritrade's director of retirement and annuities.

Additionally, millennials face a challenging job market and lackluster wage growth. About 23 percent of 18-to-34-year-olds are underemployed, according to Anthony Carnevale, director of the Georgetown University Center on Education and the Workforce. Meanwhile, student loan debt has reached record highs. On average, student borrowers who graduated this year owe more than $35,051, according to Mark Kantrowitz, a student financial aid policy expert and publisher of Edvisors.com. Paying off debt has made it tougher for many to set money aside. "Millennials — who should have plenty of time on their side when it comes to long-term investing — are starting off behind the curve. They're putting off saving for retirement because of other financial obligations, a decision that can have unfortunate consequences," Sadowsky said. In addition to pushing off saving for retirement, nearly half of millennials said they were delaying buying a house, according to the TD Ameritrade survey, which included 1,000 adults age 18 and older. Twenty-nine percent said their financial obligations prompted them to delay getting married and 38 percent said they postponed having children. Living through the Great Recession from December 2007 to June 2009 and its aftermath has also affected their attitude toward money, said Janet Stanzak, a certified financial planner and principal of Financial Empowerment in Bloomington, Minnesota. "Because of what they witnessed during the economic downturn, they are being really conscientious before they make big moves," she said. "They recognize how dangerous it is getting in over their head."