A 35-year-old female skull is examined during a medicolegal autopsy in Delhi. The forensic dentist observes moderate attrition of the occlusal surfaces of all molars, secondary dentin deposition visible on radiographs, and gingival recession with exposed root surfaces. The third molars are fully erupted and root-formed. Which of the following age-related dental changes is the MOST reliable indicator of her age in this case?