My boyfriend pledged and was initiated in his 5th year here at Tech.

His fraternity has taken in several 4th and 5th years (5+ years is perhaps more common than here than 4...lots of engineers and co-ops and such)...as long as they truly want to make the committment now, that's what matters.
Also, as a 4th year now myself, I'm a new member of the local sorority on campus, but that's a whole different ball game. I think of it this way: I needed the first few years to grow as an individual, to kind of find out who I am and want to be. Then I realized that this sorority fit into that vision.