The following are guidelines only. Please follow your veterinarians suggestions if they differ.
Cats get their adult teeth by 6 months of age. Tartar accumulates on cats teeth the same as on humans teeth. Accumulation of tartar can cause gingivitis, inflamed gums, cause bad breath, cause tooth decay (leading to tooth removal), and kidney problems. Talk to your vet about the dental needs of your cat and about the options: there are some vets who clean teeth without using anesthetics; there is a new laser technique; there are dental brushes and toothpaste if you wish to brush the teeth yourself.
Note: Tartar will only accumulate on the outside of the teeth due to cats rough tongue rubbing against the teeth.