The (mean) average age of a Senator is 61.8 years. Diane Feinstein is about to turn 87 on Monday. Chuck Grassley and Richard Shelby are right behind her at 86. (Jim Inhofe rounds out the top three at a spry 85).
Unrelated fact: the oldest-known animal is a Seychelles giant tortoise named "Jonathan," at around 187 years old.