**Core Concept:** Sucking habits, particularly finger sucking, can lead to dental malocclusion and subsequent dental problems in growing children. It is essential to interrupt such habits to protect the developing dentition and oral structures.

**Why the Correct Answer is Right:** The correct answer, **D. Before the age of 8**, is based on the understanding that permanent teeth start to erupt between the ages of 6-8 years. Interrupting finger sucking before this age helps prevent the displacement of permanent teeth caused by the continuous pressure exerted by the finger. This age is also known as the critical period of tooth eruption during which the permanent teeth have not yet emerged and are more vulnerable to displacement.

**Why Each Wrong Option is Incorrect:**

A. **7-8 years:** This option is incorrect because the critical period of tooth eruption lasts until the age of 8, not just during this specific window. Interrupting the habit before the age of 8 is crucial to prevent dental issues.

B. **Before the age of 12:** The correct answer is before the age of 8, not 12, as tooth displacement can occur during the critical period of tooth eruption, which lasts until the age of 8.

C. **After the age of 12:** This option is incorrect because the critical period of tooth eruption ends at the age of 8, not after 12 years. Interrupting the habit after this age would not prevent displacement of permanent teeth.

E. **After tooth eruption:** This option is incorrect because the habit should be stopped before tooth eruption occurs to prevent displacement. Tooth eruption typically begins at around 6 years old and continues until the age of 12-13.

**Clinical Pearl:** Interrupting sucking habits, such as finger sucking, before the age of 8 is essential to protect the developing dentition and prevent dental issues associated with prolonged sucking habits. Encouraging early intervention can help maintain proper tooth alignment and prevent the need for orthodontic treatment in the future.