**Core Concept**
The index age group for deciduous teeth in years is based on the World Health Organization (WHO) guidelines for dental caries assessment. This concept is crucial in pediatric dentistry for evaluating the prevalence and severity of dental caries among children.

**Why the Correct Answer is Right**
The correct answer is based on the WHO's recommendation to divide the age range of children into specific intervals for the purpose of epidemiological studies. The index age group for deciduous teeth is typically set at 12 years, as it represents a significant milestone in tooth development and is often used as a reference point for assessing dental caries.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not align with the WHO's recommended age intervals for dental caries assessment.
**Option B:** This option is incorrect because it is not a standard age interval used in pediatric dentistry for evaluating dental caries.
**Option C:** This option is incorrect because it is not the recommended age interval for assessing dental caries in children with deciduous teeth.

**Clinical Pearl / High-Yield Fact**
It is essential for pediatric dentists and healthcare professionals to be aware of the WHO's guidelines for dental caries assessment, including the index age group for deciduous teeth, to accurately evaluate and manage dental caries among children.

**Correct Answer: D. 12 years.**