**Core Concept**
Delayed puberty is a condition where adolescents fail to develop secondary sexual characteristics within the normal age range. The Tanner stages, a widely accepted classification system, describe the progression of physical changes during puberty. A girl is considered to have primary amenorrhea if she has not started menstruating by the age of 16 years, accompanied by a lack of secondary sexual characteristics.

**Why the Correct Answer is Right**
The primary concern in delayed puberty is the potential for underlying hormonal imbalances or other underlying medical conditions. In girls, the onset of puberty is triggered by the hypothalamic-pituitary-gonadal axis, specifically the production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). By 16 years, the absence of breast development and pubic hair, along with primary amenorrhea, suggests a need for further evaluation. The American Academy of Pediatrics recommends that girls with primary amenorrhea and absent secondary sexual characteristics should be evaluated by a healthcare provider.

**Why Each Wrong Option is Incorrect**
**Option A:** While 12 years is the age cutoff for evaluation of primary amenorrhea in girls, this option is incorrect because it is too early to diagnose delayed puberty.
**Option B:** 14 years is still within the normal range for the onset of puberty in girls, so primary amenorrhea at this age is not necessarily indicative of delayed puberty.
**Option D:** 18 years is the age cutoff for evaluation of primary amenorrhea in girls with normal secondary sexual characteristics, not those with absent secondary characteristics.

**Clinical Pearl / High-Yield Fact**
The American Academy of Pediatrics recommends that girls with primary amenorrhea and absent secondary sexual characteristics should be evaluated by a healthcare provider by the age of 16 years. This evaluation should include a thorough medical history, physical examination, and laboratory tests to rule out underlying medical conditions.

✓ Correct Answer: C. 16 years.