Yes, they can get old, but it takes a long time- they have a fairly good shelf life. Sometimes the problem after a long time is that the wax adhesive no longer works, which means there is nothing to make them stick to the surface you are applying them too, and they tend to break up as you pull the backing away.
Try putting down a few symbols on a sheet of clear plastic, and look at the back side to see how well they are sticking.