**Core Concept**
Thumb sucking is a common habit seen in infants and young children, often serving as a self-soothing mechanism or a way to cope with stress and anxiety. This behavior is considered normal in certain age groups due to its prevalence and developmental significance.

**Why the Correct Answer is Right**
The American Academy of Pediatric Dentistry (AAPD) recommends that children should stop thumb sucking by the age of 2-3 years. This is because prolonged thumb sucking beyond this age can lead to dental malocclusion (misalignment of teeth) and other orthodontic problems. The habit can also interfere with the normal development of the palate and the shape of the upper jaw.

**Why Each Wrong Option is Incorrect**
**Option A:** Thumb sucking is a normal behavior in children as young as 6 months. **Incorrect**, as this is too early for a child to exhibit thumb sucking as a self-soothing mechanism.
**Option B:** Thumb sucking is a normal behavior in children up to the age of 5 years. **Incorrect**, as this is still too early for children to stop thumb sucking without potential orthodontic consequences.
**Option C:** Thumb sucking is a normal behavior in children up to the age of 7 years. **Incorrect**, as this is still too early for children to stop thumb sucking without potential orthodontic consequences.

**Clinical Pearl / High-Yield Fact**
Parents and caregivers should be aware of the potential risks associated with prolonged thumb sucking and encourage children to stop the habit by the age of 2-3 years to prevent orthodontic problems.

**Correct Answer:** C.