**Core Concept**
The xiphoid process is a small, cartilaginous structure located at the lower end of the sternum. Fusion of the xiphoid process is a developmental process where it eventually becomes a bony structure, integrating with the rest of the sternum. This process is crucial in understanding the anatomy of the thoracic cage.

**Why the Correct Answer is Right**
Fusion of the xiphoid process typically occurs in the 40s to 50s in life. This process is a gradual one, where the cartilaginous xiphoid process gradually ossifies and becomes a bony structure, eventually fusing with the rest of the sternum. The process is mediated by the presence of osteoblasts and osteoclasts, which are involved in bone formation and resorption.

**Why Each Wrong Option is Incorrect**
**Option A:** The xiphoid process does not typically fuse with the body of the sternum at birth. Instead, it remains as a separate cartilaginous structure.
**Option B:** Fusion of the xiphoid process is not a process that occurs during childhood or adolescence. It is a late-life process.
**Option C:** The xiphoid process does not fuse with the manubrium at any stage of life. It remains a separate structure.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the xiphoid process fuses with the rest of the sternum in late life, which can be an important consideration in cases of trauma or surgical procedures involving the thoracic cage.

**Correct Answer: D. 60s**