**Core Concept**
Infantile colic is a condition characterized by episodes of crying, abdominal distension, and drawing up of the legs in infants, typically presenting in the first few weeks of life. The exact cause of colic remains unknown, but it is thought to be related to gastrointestinal tract immaturity, overstimulation, or sensitivity to certain foods.

**Why the Correct Answer is Right**
Colic generally disappears by the age of 3-4 months as the infant's gastrointestinal tract matures. The development of the gut and the maturation of the enteric nervous system are thought to play a significant role in the resolution of colic. By 3-4 months, most infants have developed sufficient gut motility and reflexes to manage their bowel movements more efficiently.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because colic can persist beyond 6 months of age in some infants.
**Option B:** This is incorrect because colic is not typically associated with a specific age range of 9-12 months.
**Option C:** This is incorrect because colic is not a condition that resolves by 18 months of age.

**Clinical Pearl / High-Yield Fact**
It's essential to note that colic does not indicate any underlying serious medical condition, and most infants grow out of it by 3-4 months of age. Parents can try various soothing techniques, such as the "5 S's" (swaddling, shushing, side/stomach position, shaking, and sucking), to help alleviate colic symptoms.

**Correct Answer:** C.