The efficacy of tooth whitening procedures depends on several key factors.

1. Type of discoloration: Different types of stains respond differently to bleaching agents. Extrinsic stains, caused by dietary factors or smoking, typically respond better to whitening treatments than intrinsic stains, which are deeper and may be due to trauma, medications, or developmental conditions.

2. Concentration of bleaching agent: The strength or concentration of the bleaching agent (commonly hydrogen peroxide or carbamide peroxide) directly affects whitening efficacy. Higher concentrations generally yield faster and more noticeable results, though with increased risk of sensitivity.

3. Duration of exposure: The length of time the bleaching agent remains in contact with the tooth surface significantly influences the degree of whitening. Longer exposure allows more penetration and breakdown of staining molecules.

4. Patient’s age: While age may influence baseline tooth color and enamel thickness, age itself does not significantly affect the chemical efficacy of the whitening agent. Older patients may have more intrinsic discolorations or enamel wear, which can influence perceived outcomes but age alone is not a limiting factor for bleaching efficacy.

Therefore, the correct answer is "Patient's age", as it is not a direct factor affecting the chemical or physical efficacy of tooth whitening procedures.

Reference:
Sturdevant's Art and Science of Operative Dentistry, 7th Edition, Chapter 16: Tooth Whitening, Page 650-660