## Core Concept
The eruption of milk teeth, also known as primary teeth or deciduous teeth, typically begins around 6 months of age. This process continues until the child is about 3 years old, with the first set of teeth being the central incisors.

## Why the Correct Answer is Right
The first milk teeth to erupt are usually the lower central incisors, which typically appear between 6 to 10 months of age. This timeline can vary slightly among infants, but 6 months is a commonly cited average for the start of teething.

## Why Each Wrong Option is Incorrect
* **Option A:** 3 months is too early for the eruption of milk teeth. At this age, babies are still adjusting to the outside environment and usually do not start teething.
* **Option B:** 12 months is relatively late for the first milk teeth to appear. By this age, several teeth might have erupted.
* **Option D:** 18 months is significantly late for the first milk teeth to appear. At this stage, most children already have several erupted teeth.

## Clinical Pearl / High-Yield Fact
A useful clinical correlation to remember is that teething often starts around 6 months of age, and the sequence of tooth eruption can be a helpful indicator of a child's developmental progress. The typical sequence starts with the lower central incisors, followed by the upper central incisors, then the lateral incisors, and so on.

**Correct Answer: C. 6 months.**