**Core Concept:**
The question pertains to dental anomalies caused by prolonged thumb or finger sucking habits in children, which can lead to malocclusion or improper alignment of teeth. This condition is known as "digit sucking habit" or "digit bite," and it can result in proclined upper anterior teeth, retroclined lower anterior teeth, and tongue thrusting.

**Why the Correct Answer is Right:**
The correct answer (D) refers to the typical duration for digit sucking habits to resolve on their own. In most cases, thumb or finger sucking habits are self-limiting and resolve as the child grows and their teeth and jaw develop. However, in certain cases, these habits may persist, leading to dental anomalies as described in the question.

**Why Each Wrong Option is Incorrect:**
A. 6-year-old children typically do not outgrow their habits on their own, leading to incorrect information.
B. Outgrowing habits by age 12 is not a specific age range, making it an incorrect answer choice.
C. The statement "before 18 months" is too early, as the habits often persist beyond this age.

**Clinical Pearl / High-Yield Fact:**
Since digit sucking habits can cause dental anomalies, it is essential for parents and healthcare professionals to monitor children's habits and encourage them to discontinue sucking their thumbs or fingers before the age of 2 years. This ensures that the child's dentition develops normally, and the child's teeth and jaw align correctly, preventing the need for orthodontic interventions in the future.

**Correct Answer:** D. Before the age of 3 years