Aging
The enamel on our teeth wears away as we age, exposing the dentin. In addition, dentin is colored by compounds called chromogens, which give it a yellow color and make it extra sensitive to staining. The longer teeth are exposed to the elements, the more the color of dentin comes through. When the tooth’s surface is cleaned, the stain becomes more apparent by contrast, so older teeth tend to look especially yellow after being cleaned.
According to Dr. John Friedewald, an orthodontist and periodontist in North Carolina, this can make people think their teeth are getting darker as they age.
The same process of aging can also cause gums to recede from the teeth, making them appear shorter and causing more exposure to the tooth surface for staining by external elements such as tobacco or wine.
