## Core Concept
The human brain grows rapidly during childhood and adolescence, with significant changes in size and structure. Understanding brain development is crucial for pediatric neurology and developmental psychology. Brain growth is often measured in terms of achieving adult size or volume.

## Why the Correct Answer is Right
By the age of 5 years, the human brain achieves approximately 90% of its adult size. This rapid growth during early childhood is critical for cognitive and motor development. The brain's growth rate slows down significantly after this period, but it continues to mature and develop throughout adolescence and into young adulthood.

## Why Each Wrong Option is Incorrect
* **Option A:** 2 years - This is too early; while the brain grows rapidly in the first two years, it does not reach 90% of adult size by this age.
* **Option B:** 10 years - This is slightly older than necessary; most of the brain's growth spurt occurs earlier.
* **Option D:** 15 years - This is too late; by 15 years, most individuals have achieved full brain maturity, far exceeding 90% adult size.

## Clinical Pearl / High-Yield Fact
A key point to remember is that brain growth and development are most rapid in the first few years of life. By age 5, 90% of adult brain size is achieved, but brain maturation, including myelination and synaptogenesis, continues into the mid-20s. This timeline is crucial for understanding developmental milestones and potential vulnerabilities.

## Correct Answer: C. 5 years