Insider and Morning Consult surveyed 2,096 Americans about their financial health and habits, 670 of whom were millennials between the ages of 23 and 38.

A solid 40% of millennials aren't in a serious relationship and have never been married, compared to just 20% of Gen Xers and 13.5% of baby boomers surveyed who said the same. Millennials are waiting longer to get married, and as Business Insider's Hillary Hoffower reports, one of the reasons is money.

An increase in the cost of college, and therefore an increase in student loans, has many millennials starting their lives in the red. Hoffower reports that millennials want to be more financially stable before they undertake the traditional milestones of adulthood, so they're marrying, buying homes, and having children later.

But among those millennials who are married, money continues to be a problem. Of married millennials, 21% said that money causes a lot of stress in their relationship, and 41% say that it causes some stress, making a total of 62% that report some level of relationship stress about money.

Read more: More than half of millennials have credit-card debt, and it reflects a financial reality other generations didn't face

It seems that money stress is just as bad for millennials living with a partner — 30% said money caused a lot of stress, while 35% said it caused some stress, for a total of 65% of cohabitating millennials reporting some level of money stress.

There are a few reasons why millennials are so worried about money. They're not only facing struggles with student loans, but also increased costs of living and stagnant wages after college. The same Insider and Morning Consult survey shows that over half of millennials have credit card debt, and Business Insider's Hoffower reports that it's likely tied to higher living costs and slow-to-rise earnings. Additionally, as Business Insider's Tanza Loudenback reports, 53% of millennials say that they're not earning enough to start saving for retirement.

But, it doesn't seem like the problem gets better as people get older. At least, not for a while.

About 50% of Gen X respondents in this survey were married, and 52% of those married Gen Xers (defined as ages 39 to 54) reported that money was a source of stress in their relationship. Of that group, 14% said money caused a lot of stress. Among Gen Xers who live with a partner, 56% reported that money caused some level of stress in their relationship.

Read more: Millennials and Gen X are both stressed, broke, and in debt — but Gen X is more worried about it

Nearing retirement and caught supporting both their aging parents and their adult children, Gen Xers report that they're not feeling like they're on solid ground financially. Hoffower reports that, according to Morning Consult and Insider's data, 41% of Gen X said that they'd define their financial health as not very good or not good at all.

For the record, things aren't quite as dire for the next-older generation. A solid 64% of married baby boomers said that money wasn't a source of stress for them, and two-thirds of boomers living with a partner said it didn't cause them stress.