**Core Concept**
The corticospinal tracts are a group of nerve fibers in the spinal cord that play a crucial role in motor control and coordination. Myelination of these tracts is a critical process that enables the transmission of impulses along the spinal cord. The timing of myelination is essential for the proper development of motor functions.

**Why the Correct Answer is Right**
The corticospinal tracts are fully myelinated by around 2 years of age. This process is facilitated by the presence of oligodendrocytes, the myelinating cells of the central nervous system. Myelination allows for faster conduction of impulses along the axons, which is vital for the development of precise motor control. The corticospinal tracts are responsible for transmitting signals from the brain to the spinal cord, and full myelination enables the coordination of complex movements.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the corticospinal tracts are not fully myelinated at birth. While some myelination begins in utero, it is not complete until several years after birth.
* **Option B:** This option is incorrect because the corticospinal tracts are not fully myelinated in adulthood. While some myelination may continue throughout life, the corticospinal tracts are generally fully myelinated by early childhood.
* **Option D:** This option is incorrect because the corticospinal tracts are not fully myelinated in premature infants. Premature infants may have incomplete myelination of the corticospinal tracts, which can lead to motor delays and other developmental issues.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the timing of myelination is critical for the proper development of motor functions. Delayed or incomplete myelination can lead to motor delays, weakness, or other neurological issues. A thorough understanding of the development of the corticospinal tracts is crucial for diagnosing and managing neurological disorders in children.

**Correct Answer:** C.