?
The negatives appear low in contrast and darker that they should be (is that a sign of overdevelopment?).
I didn't quite follow that sentence....did you mean that the
prints are low in contrast?
With old paper
and developer, plus the variables in neg and enlarger exposure as mentioned in the previous reply, it might be difficult to pin down where the problem lies.
Why not try the paper with a known good neg and your usual fresh developer and printing conditions. If that works, you obviously know the condition of the paper...or you can experiment with different times, etc., if the paper is past its best.
Then, conversely, try the old developer with some of your usual fresh paper...again you are then experimenting with only one variable.
(At a guess, I'd think that the developer could be the problem. I'd not trust 6-year old chemicals, but paper of the same age, if well stored, should be reasonably usable?)