Johnson will be 68 by January 2015, so his retirement is another stop towards a somewhat less aged Senate.
In fact, we already have the three most likely replacements already in place. Former Governor Mike Rounds (b. 1954) is already running and is the probable Republican nominee, although we've learned not to count on such things always working out for the GOP. On the Democratic side, either Johnson's son Brendan Johnson (b. 1975) or Stephanie Herseth Sandlin (b. 1970) will probably make the race, perhaps setting up a tough primary. Yes, this is could be yet another example of a relatively old Republican nominee.
As political junkies will have noticed, this also could be a setback for ridding the Senate of dynastic politicians. Tim Johnson was not a dynastic politician, at least not as far as I can tell. Of course his son would be. And so would Stephanie Herseth Sandlin -- in fact, she's a third generation South Dakota politician. Mike Rounds also comes from a political family; his father (according to wikipedia) had several jobs in state government, including state director of highway safety. I haven't looked at the other retiring Senators, other than noting that Jay Rockefeller (obviously a dynastic politicians) may be replaced by Shelley Moore Capito, whose father was governor of West Virginia. Oh, and she was born in 1953, so that's going on, too.