Roll Call has published a list of the 113th Congress organized by seniority for both the Senate and House. Chuck Grassley is in sixth place (up from 11th in 2009) and now the only senator left who was originally elected in 1980. Tom Harkin is in eighth, with only John Kerry between them.
The good side of this is that Iowa has the most clout in the Senate right now. The bad side is that when the senators are replaced, that clout will take a big tumble.
In the House, Tom Latham is ranked 82nd out of 435, barely in the top fifth.