Teeth may start to turn yellow during the 20s and 30s. Dr. Dorfman explains, “Teeth are slightly porous and, over your lifetime, will pick up stain. If you smoke or drink a lot of dark liquids like coffee and tea, over your lifetime, your teeth will tend to pick up the pigmentation and stain from these products. So, it’s really dependent on how frequently you expose your teeth to external sources that will stain them. On the other hand, some people’s teeth tend to resist stains more than others, so it’s literally a case-by-case scenario. But, most people tend to notice that their teeth are starting to yellow in their 20s and 30s, and that’s a good time to start using any kind of tooth whitening product.”