There can be two reasons for yellow teeth. Firstly enamel is the outermost covering of the teeth, while dentin is the underlying yellow layer. The thinner your enamel is, the more the yellow color of the dentin shows through. Hence, aging can cause discoloration due to the thinning of the enamel. Aggressive and hard brushing can also strip away the enamel layer of your teeth over time and make your teeth appear yellow. Secondly, the plaque that resides on the surface of your teeth turns yellow over a period of time because of the stains it picks up from the food we eat.