There are many reasons why teeth can become discolored. The major reasons are:
Diet – Drinking tea, coffee, red wine or other food and drink with strong colorings.
Smoking – Smoking can result in teeth appearing a yellowish color.
Age – As people grow older their teeth can become darker naturally.
Tooth decay, fillings and tartar build-up can cause discoloration.
Trauma – Teeth have nerves and blood vessels inside them. If these nerves and blood vessels are damaged the tooth may become darker – even several years after the trauma.
Medication – Staining can be caused by diseases or medicines. Tetracycline, an antibiotic, will cause a distinct discoloration if given to a child whose adult teeth are still developing.