Why teeth turn yellow
Teeth turn yellow for two reasons, both of which tend to accelerate with age:
Enamel thinning
Baking soda may help to whiten teeth that have yellowed.
The outer layer of teeth consists of enamel, which is colored almost white and protects the deeper tooth structure. Beneath the enamel is a layer of tissue called dentin, which is yellow-brown. When the enamel layer thins or wears away, the teeth begin to look darker.
Acidic foods, gum disease, and aging can wear down tooth enamel. Some people also have enamel that is naturally thinner.
Stains
Specific foods and beverages, such as coffee, can stain the teeth. Some foods that stain the teeth may also wear down enamel, increasing the yellowing.
Other sources of stains include smoking and tobacco products and certain types of antibiotics.