The problem with passing term limits is that it completely changes the dynamic of how Congress functions in ways that are not to any of the incumbents' clear advantage, and to the obvious disadvantage of most of them. Suppose you are one of the Senators or Reps from an out-of-the-way, not-particularly-important State or district. In a term-limited Congress, you will never acquire a prominent position as a committee head and have relatively little chance of even being seated on one of the major committees (Justice, Defense, Banking, etc.). In the current system, your longevity will eventually be rewarded in a seniority system. Where is the incentive to dump your current standing in a known system and screw yourself (and also the people you represent, for whom you will NEVER be in a position to bring home the bacon and possibly improve their relative importance in the national scheme)?
After watching Byrd, Spector, and company dither around, though, an age limit wouldn't trouble me though.