**Core Concept**
The Schour-Massler classification is a developmental classification used to describe the eruption of primary and permanent teeth. It is essential for pediatric dentistry and orthodontics to understand the timing and sequence of tooth eruption. The classification focuses on the morphological changes in the tooth during its development.

**Why the Correct Answer is Right**
According to the Schour-Massler classification, the crown completion of both permanent maxillary and mandibular first molars is at the age of 12-16 years. This classification helps in understanding the developmental stages of the tooth, including the initiation, growth, and completion of the crown. The permanent first molars are one of the primary teeth that erupt during the mixed dentition stage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the correct age range for the crown completion of permanent first molars.
**Option B:** This option is incorrect as it does not accurately reflect the Schour-Massler classification.
**Option C:** This option is incorrect as it does not provide a valid age range for the crown completion of permanent first molars.

**Clinical Pearl / High-Yield Fact**
The Schour-Massler classification is a useful tool for understanding the developmental stages of tooth eruption, but it is essential to note that the timing of tooth eruption can vary depending on individual factors.

**Correct Answer:** C. 12-16 years