Enamel is the only thing dentists can bleach. We cannot bleach cementum, and we cannot bleach dentin (the majority of the tooth’s underneath material). Dentin’s natural color is yellow and the enamel covering the dentin is anywhere from a fraction of a millimeter thick up to maybe as much as two millimeters thick which means some of that yellow tint may be seen through the enamel.
As the tooth gets older, the nerve will tend to recede and get smaller and smaller, and as it recedes, it lays down what’s called secondary dentin, which tends to be more yellow than primary dentin.
So your teeth are getting darker over time because of the combination of what the enamel has absorbed over time and the color of the dentin that lies underneath the enamel. This is why teeth whitening, or tooth bleaching, can be very effective. It bleaches out those stains that have been absorbed into the enamel, and you can basically return the color of the enamel back to what it was when you were a kid. In some cases, the teeth actually turn lighter than there were when you were a kid.