**Core Concept**
The lens of the eye is composed of cells that undergo a process called fiber cell differentiation, where young cells are produced at the equatorial region and mature into lens fibers. The oldest cells in the lens are located at the **nucleus**, which is the central, dense region of the lens.

**Why the Correct Answer is Right**
The oldest cells in the lens are indeed located at the nucleus. This is because the process of fiber cell differentiation occurs at the equatorial region, and as the cells mature and move centrally, they become compressed and form the dense nucleus. The nucleus is composed of highly compacted, older lens fibers that are responsible for the lens's refractive properties.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the **epithelium** is the outermost layer of the lens, composed of young cells that are responsible for lens growth and maintenance, not the oldest cells.
**Option B:** This option is incorrect because the **cortical** region of the lens is composed of older lens fibers, but it is not the oldest region.
**Option C:** This option is incorrect because the **capsule** is the outermost fibrous layer of the lens, not a region of cells.

**Clinical Pearl / High-Yield Fact**
The lens becomes more rigid and less flexible with age due to the compaction and aging of the lens fibers in the nucleus.

**Correct Answer: C. Nucleus**