**Core Concept**
The maxillary sinus is one of the four paired paranasal sinuses that play a crucial role in the drainage and ventilation of the nasal cavity. The development and growth of the maxillary sinus are closely related to the eruption of the permanent teeth, particularly the first molar.

**Why the Correct Answer is Right**
The maxillary sinus achieves its maximum size around the age of 18-25 years, when the first molar has erupted. This is because the growth of the maxillary sinus is influenced by the surrounding bone and the eruption of the permanent teeth. The maxillary sinus expands to accommodate the root of the first molar, which helps to increase its volume and surface area. This process is essential for the proper drainage and ventilation of the nasal cavity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not correct because the maxillary sinus does not achieve its maximum size during childhood.
**Option B:** This option is not correct because the maxillary sinus does not reach its maximum size immediately after birth.
**Option C:** This option is not correct because the maxillary sinus continues to expand and grow until the eruption of the permanent teeth.

**Clinical Pearl / High-Yield Fact**
The maxillary sinus is the largest of the paranasal sinuses and plays a crucial role in the drainage and ventilation of the nasal cavity. Its growth and development are closely related to the eruption of the permanent teeth, particularly the first molar.

**Correct Answer: C. 25 years**