Teeth tend to get darker as we age. To understand this you need to understand the anatomy of the tooth. The most outer layer of your tooth is enamel. This is the white protective layer on the surface of young and healthy teeth. Under the enamel is dentin. Dentin is much softer than enamel and has a yellow tint to it. Finally, the very inside of the tooth is the pulp. The pulp is made up of the nerves and blood supply to your tooth.
As we age the enamel on our teeth tends to thin out, exposing more of the yellow dentin color. Additionally, the pulp of our teeth starts to get smaller as the dentin takes over where the pulp once was. When more dentin is formed, this dentin tends to be darker than the dentin originally in our teeth. These factors combined lead to darker teeth as we get older.