**Core Concept**
Aging periodontium refers to the changes that occur in the periodontal tissues, including the gingiva, periodontal ligament, cementum, and alveolar bone, as a result of the aging process. These changes can lead to a range of clinical manifestations, including periodontal disease.

**Why the Correct Answer is Right**
During aging, the periodontal ligament undergoes a process called "ligament degeneration," characterized by loss of collagen fibers and a decrease in ligament thickness. This leads to a decrease in the attachment of the tooth to the surrounding bone, making it more susceptible to periodontal disease. The gingiva also undergoes changes, including thinning, decreased vascularity, and reduced resistance to plaque accumulation. Additionally, the cementum and alveolar bone undergo changes, including decreased cementum thickness and alveolar bone loss.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe a characteristic of aging periodontium.
**Option B:** This option is incorrect because it describes a feature of periodontal disease, not aging periodontium.
**Option C:** This option is incorrect because it describes a feature of periodontal disease, not aging periodontium.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that aging periodontium is a risk factor for periodontal disease, and regular dental check-ups and proper oral hygiene practices are crucial for maintaining periodontal health in older adults.

**Correct Answer: D. Decreased periodontal ligament thickness**