It would be best for a veterinarian to look at her teeth and determine whether or not they need a cleaning. It may just be staining but it also may be tartar build up. If she does need a cleaning she would have to be put under anesthesia and that can be risky at her age.(It can be a risk at any age but the older a cat is the more of a risk)
If he feels it's not necessiary for a cleaning then he can recomend a few products that will help keep her teeth looking good. Dental health is very important and if the teeth are not properly taken care of it can lead to many serious problems.
I once had a cat whose teeth needed to cleaned when she was 13 years old. The vet used a gas anesthesia on this cat to reduce the risks, and the cat came through the procedure extremely well. When I picked my cat up later that same day, she was completely recovered from the effects of the anesthesia.
I also paid a little more and had blood work done on this cat prior to anesthesia, I really felt it was worth the extra expense.
Gas anesthesia is great. :tounge2: They come out of it so quickly and they bounce back easier and act as though nothing ever happened. Although it can be extremly difficult to gas down a kitty who dosen't want to be.