**Core Concept**
Delayed dentition refers to the failure of teeth to erupt within the expected time frame, which is crucial for diagnosing developmental and genetic disorders. The primary concern is the timing of the eruption of primary teeth, which usually starts around 6 months of age and continues until the child is about 3 years old.

**Why the Correct Answer is Right**
The correct answer is 3 years. This is because primary dentition typically erupts between 6 months to 3 years of age. If no teeth have erupted by the age of 3 years, it is considered delayed dentition and warrants further evaluation. The primary teeth are essential for speech development, chewing, and proper alignment of the permanent teeth.

**Why Each Wrong Option is Incorrect**
**Option A:** 6 months - This is the age range when primary dentition starts to erupt, not when it should be fully erupted.
**Option B:** 12 months - Although some primary teeth may still be erupting at 12 months, this is not the age when all primary teeth should have erupted.
**Option C:** 5 years - This age is beyond the typical range for primary dentition, which usually erupts between 6 months to 3 years of age.

**Clinical Pearl / High-Yield Fact**
Delayed dentition can be associated with various genetic and developmental disorders, such as Down syndrome, cleft palate, and hypothyroidism. A thorough evaluation, including a detailed medical and dental history, is essential for diagnosing the underlying cause.

**Correct Answer:** C. 3 years.