(CNN) Millennials are a bit of a mystery to Janet Yellen.

The head of the U.S. Federal Reserve said Tuesday that the behavior of millennials — which typically refers to a generation of people born in the 80s and 90s — has top economists scratching their heads.

"I think we're just beginning to understand how the millennials are behaving," Yellen said before the Senate Banking Committee. "They're certainly waiting longer to buy houses; to get married. They have a lot of student debt. They seem quite worried about housing as an investment. They've had a tough time in the job market."

As the economy continues to gain strength, Yellen said she expects more millennials to buy homes and start families. "But," she quipped, "we've yet to really see how this is going to affect that generation."

Tuesday kicked off Yellen's two-day back-to-back testimony on Capitol Hill this week. For the first time since becoming head of the U.S. central bank last February, Yellen is facing a Congress that's controlled by Republicans in both the House and the Senate.

Read More