**Core Concept:** The concept being tested is the balance between extracellular fluid (ECF) and intercellular fluid (ICF) in the human body, and when they become equal.

**Why the Correct Answer is Right:** At birth, the human body consists primarily of extracellular fluid (ECF), which includes plasma, lymph, and interstitial fluid. As an individual grows and develops, there is an increase in the volume of intracellular fluid (ICF), which includes the fluid within cells, mitochondria, and other organelles. The balance between ECF and ICF changes as cells grow and divide, leading to an increase in ICF volume.

In infants, the ratio of ECF to ICF is approximately 2:1. This ratio gradually shifts towards 1:1 as the infant grows and develops. As a result, by the age of 20-25 years, extracellular fluid is equal to intercellular fluid.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because ECF and ICF are not equal in adults. The correct answer is based on the growth and development process, not a specific age.

B. This option is incorrect because it is a generalization for all ages, not specific to the age at which ECF equals ICF.

C. This option is incorrect because it is a generalization for all ages, not specific to the age at which ECF equals ICF.

D. This option is incorrect because it is a generalization for all ages, not specific to the age at which ECF equals ICF.

**Clinical Pearl:** The understanding of the change in ECF and ICF balance during growth and development is crucial for healthcare professionals, especially pediatricians, intensivists, and nephrologists. This knowledge is essential for accurate assessment of fluid balance, electrolyte homeostasis, and overall health monitoring in infants, children, and adolescents.