The short version, if you only read one thing
Teeth darken with age through three main mechanisms: enamel thinning from gradual wear and erosion makes the underlying dentin more visible; dentin darkening as the dentin layer naturally becomes more yellow with age; and accumulated surface staining from food, drink, and tobacco over decades. Whitening addresses the surface staining directly and lightens the dentin layer underneath; it does not restore enamel thickness. This means whitening can produce meaningful colour improvement but cannot return teeth to a youthful brightness if substantial enamel has been lost. Naturally yellow tooth colour (predominantly yellow tones) responds well to bleaching at any age; naturally grey tones respond poorly. Understanding which type of darkening is actually affecting your teeth helps set realistic expectations for what whitening can deliver.
Enamel thinning — the structural change you can see
Enamel is the white, hard outer layer of every tooth. New teeth have enamel between 2-2.5 mm thick at the biting edges and 1-1.5 mm thick on the facial surfaces. Over decades, this layer gradually thins through normal wear from chewing, abrasion from toothbrushing, and chemical erosion from acidic foods and drinks.
The visual effect is twofold. First, the natural enamel itself is the brightest layer — slightly translucent but generally white. As it thins, the tooth looks less bright simply because less of the brightest material remains. Second, the layer underneath (dentin) becomes more visible through the thinned enamel. Dentin is naturally yellower than enamel, so as enamel thins the apparent colour of the tooth shifts toward yellow.
This is the part of tooth ageing that whitening cannot directly reverse. Enamel does not regrow; thinning is permanent. Whitening can lighten the dentin layer (which makes the tooth look brighter through the thinned enamel) but cannot restore the original enamel thickness.
3 mechanisms
Three distinct mechanisms drive age-related tooth darkening — enamel thinning, dentin darkening, and accumulated surface staining. Whitening addresses two of three effectively (dentin and surface) but cannot restore lost enamel. Understanding which mechanism dominates in your case helps set realistic expectations for what bleaching can deliver.
Dentin darkening — the layer underneath getting yellower
Dentin makes up the bulk of every tooth — the layer between the enamel and the pulp chamber containing the nerve. Dentin is naturally yellower than enamel and contains microscopic tubules that connect the outer surface to the inner pulp. Over decades, the dentin gradually darkens through several mechanisms.
The pulp chamber inside the tooth gradually fills with secondary dentin produced by the tooth in response to ageing and minor trauma. This secondary dentin is denser and darker than the original dentin. As the pulp chamber shrinks and the dentin layer grows thicker and darker, the overall tooth colour shifts toward yellow or grey.
The dentin tubules can also accumulate pigments over years, and the proteins within dentin can change slightly in colour with ageing. The combined effect is that the colour underneath the enamel gradually darkens.
Whitening addresses this layer reasonably well. The peroxide molecules in bleaching gels diffuse through enamel into the dentin and oxidise the dark organic compounds that have accumulated. The result is a meaningful lightening of the underlying colour, which shows through the enamel as a brighter overall tooth appearance.
Read also
The full pillar on whitening procedures — what bleaching agents do, the difference between professional and consumer products, sensitivity prevention.
Surface staining — the most visible and most reversible
The third mechanism is the accumulation of staining substances on and within the enamel surface over years. Coffee, tea, red wine, dark berries, soy sauce, turmeric, tobacco, and many other substances contain pigments that bind to enamel and accumulate over time.
Some of this surface staining is removed by routine brushing and professional cleanings. Some — particularly stains that have worked into the microscopic surface roughness of enamel — accumulates beyond what surface cleaning addresses. After decades of coffee consumption, a meaningful fraction of the visual darkening is from this accumulated surface and near-surface staining.
Whitening addresses surface staining most directly and most effectively. The peroxide-released oxygen breaks down the staining pigments at and near the surface, producing immediate visible improvement. This is why heavy coffee drinkers often see dramatic results from whitening — much of their discolouration is the most reversible kind.
Patients who consume fewer staining substances see less dramatic improvement from whitening because they have less of the most reversible component to address.
