**Core Concept**
The distribution of body water between the intracellular fluid (ICF) and extracellular fluid (ECF) compartments changes significantly during infancy and childhood. This shift is crucial in understanding various physiological and clinical processes, such as fluid balance and electrolyte regulation.

**Why the Correct Answer is Right**
The ratio of ICF to ECF volume reaches adult values by around 1 year of age. This is due to the reduction in ECF volume as a percentage of total body water, which occurs as a result of increased ICF volume. At birth, the ECF accounts for approximately 20% of total body water, whereas by 1 year of age, this percentage decreases to around 15%. This change is largely attributed to the expansion of the ICF compartment, which is driven by the growth and development of cells and tissues.

**Why Each Wrong Option is Incorrect**
**Option B:** 2 years - This is incorrect because the ratio of ICF to ECF volume continues to shift and mature until around 1 year of age.
**Option C:** 3 years - This is incorrect because, although the ICF to ECF ratio is closer to adult values by 3 years, the exact timing is around 1 year of age.
**Option D:** 4 years - This is incorrect because the ICF to ECF ratio is already close to adult values by 1 year of age, and there is no significant shift between 1 and 4 years.

**Clinical Pearl / High-Yield Fact**
It is essential for pediatricians and neonatologists to understand the changing distribution of body water during infancy and childhood to provide accurate fluid management and electrolyte replacement therapy, especially in critically ill patients.

**✓ Correct Answer: A. 1 year**