From three years ago:
The flip side of this was in 2008, when 84-year-old Frank Lautenberg ran for reelection in New Jersey. That was a Democratic year when the Democrats might’ve done well with just about anybody. (Or maybe not; I don’t really follow New Jersey politics and am just extrapolating from national polls.) When 2014 rolls around, they’re going to need to find someone new, and at that point they might wish they had an incumbent already in the slot.
What makes sense for the individual officeholder—stay in when you think you’ll have an easy win, and wait to quit when the going is getting tough—isn’t so helpful for the national party.