There are a few ways to tell how old a door knob is. One way is by looking at the finish. If the finish is worn or discolored, it is likely an older door knob. Another way to tell the age of a door knob is by the style. Older door knobs tend to be more ornate and detailed than newer ones. Finally, you can sometimes tell the age of a door knob by the materials it is made of. Brass and bronze are common materials for antique door knobs, while glass or plastic are more likely to be found on newer models.