Guzheng soundboards are mostly paulownia, because it is soft with an even grain, transmits vibration readily and lets the instrument speak easily.
Frames, rims and backboards usually use hardwoods. They provide support and stability, and harder timber helps hold the structure true.
Selection and preparation of the soundboard has a large effect on tone. The wood must be dried, naturally or in a kiln, to bring its moisture content into range — otherwise the finished instrument will distort as conditions change.
So-called old timber generally means wood stored for a long time, with a stable moisture content and internal stresses largely released. Soundboards made from it tend to be more stable, but old is not automatically better than new: selection and workmanship decide in the end.
Thickness, grain density and the presence of knots all show up in the tone. Rather than judging by the label, play the instrument and decide whether the sound is what you want.