**Core Concept**
The Infant Mortality Rate (IMR) is a measure of the number of deaths of infants under one year of age per 1,000 live births in a given year. It is a critical indicator of a country's healthcare system and access to quality medical care.

**Why the Correct Answer is Right**
The age of infants is considered in IMR because it reflects the risk of death during the first year of life, which is a vulnerable period due to various factors such as birth asphyxia, congenital anomalies, infections, and accidents. The World Health Organization (WHO) defines the first year of life as the period from birth to 12 months (0-12 months). This age range encompasses the neonatal period (0-28 days), early infancy (29 days-1 year), and late infancy (1-12 months).

**Why Each Wrong Option is Incorrect**
* **Option A:** The age of infants is not considered in IMR, which is incorrect because IMR specifically measures the risk of death during the first year of life.
* **Option B:** The correct age range for IMR is not specified, but it is generally considered to be 0-12 months, not 0-18 months or any other range.

**Clinical Pearl / High-Yield Fact**
One important aspect of IMR is that it is influenced by various factors, including maternal health, prenatal care, birth weight, and access to healthcare services. A high IMR can indicate a lack of access to quality medical care, poor maternal health, and inadequate healthcare infrastructure.

**Correct Answer:** C. 0-12 months.