It looks like it could be Michael Bennet, who I believe was born in 1965. He's the school superintendent in Denver, and was rumored as an Education Secretary possibility. According to widespread reports he will be named tomorrow as the replacement for Senator Ken Salazar, presuming that Salazar is confirmed as Obama's Interior Secretary, which should be a gimme.
I don't know a whole lot about Bennet, other than that he got a fine Connecticut education via Wesleyan and Yale, and he made money in law and business before becoming the Denver mayor's chief of staff and then ed chief.
But if I'm right about his age, he will be the first legitimate GenXer in the Senate. I kind of count John Sununu, who was born in '64 and has always struck me as pretty post-Boomer -- but of course, he's been ousted now in favor of full-fledged, born-in-'47 Boomer Jeanne Shaheen, so there you go. The youngest will now be Mark Pryor, 1963. Mark Begich of Alaska, the youngest of the newly elected Senators, was born in '62.