**Core Concept**
The pubic symphysis is a cartilaginous joint that is particularly susceptible to changes in its morphology with age, making it a valuable indicator for estimating an individual's age, especially during adulthood. This is due to the natural process of degeneration and remodeling that occurs in the joint over time.

**Why the Correct Answer is Right**
The pubic symphysis undergoes significant changes throughout a person's life, including the formation of osteophytes (bone spurs), changes in the joint surface, and the appearance of a bony bridge between the two pubic bones. These alterations are a result of the continuous process of wear and tear, and they can be used to estimate an individual's age with a reasonable degree of accuracy. In particular, the development of a bony bridge is a key indicator of aging, as it typically appears in individuals between 20-50 years of age.

**Why Each Wrong Option is Incorrect**
**Option A:** The skull, while useful for estimating age in infants and young children, is less reliable for individuals between 20-50 years due to the presence of multiple growth plates and the potential for trauma or other external factors to affect its morphology.
**Option B:** Ribs are not typically used for age estimation in adults, as their morphology can be influenced by a variety of factors, including disease and trauma.
**Option C:** The sternum, while useful for age estimation in younger individuals, is not as reliable for individuals between 20-50 years due to the presence of a single growth plate, which can make it more difficult to accurately estimate age.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the pubic symphysis should be examined in conjunction with other skeletal features to achieve a more accurate estimate of age. Additionally, the presence of any pathological conditions or trauma should be taken into account when interpreting the morphology of the joint.

**✓ Correct Answer: D. Pubic symphysis**