I wouldn't, even a "really old" piece will probably have a lot of oils left in it, I have seen rails from a 100+ year old fence that looked just like new on the inside when cut, IMHO cedar is a bad choice for an aquarium. The natural oils would most likely have some very adverse effects, and if not it is a softer wood that will rot under water.
I wouldn't use cedar of any age. The oils are dangerous to animals. When I was about 10 used a piece in my firebelly toad's aquarium and almost killed him. I would think that fish would be just as sensitive to it as my toad was. I don't know if they lose their oils over time, but I wouldn't take the chance- esp in an aquarium where there's been $$ invested in fauna. It would really suck to come home to a tank full of sick or dead fish.