## Core Concept
The paranasal sinuses are air-filled cavities around the nasal cavity that develop and pneumatize at different stages of human growth and development. Their aeration and development are critical for understanding various clinical conditions.

## Why the Correct Answer is Right
The sphenoid sinus is known to pneumatize and start aeration relatively late in childhood, around 2-3 years of age or even later. This sinus develops within the sphenoid bone and its aeration process continues to expand throughout childhood and adolescence.

## Why Each Wrong Option is Incorrect
* **Option A:** The maxillary sinus is one of the earliest paranasal sinuses to develop and is aerated at birth. Therefore, it does not fit the description of sinuses that aerate after birth in the context of late pneumatization.
* **Option B:** The ethmoid sinuses begin to develop and aerate early in life, with some sources suggesting their presence at birth or shortly thereafter. They are not the best example of sinuses aerating significantly after birth.
* **Option D:** The frontal sinus typically begins to pneumatize around 6 years of age and may not be fully developed until late adolescence. However, the question seems to focus on sinuses that start aeration "after birth" in a broader sense, and given the timeline, the sphenoid sinus fits better as it starts aerating a bit later.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the development and aeration of paranasal sinuses are crucial for understanding the spread of infections and the management of sinusitis in children. The sphenoid sinus's late development makes it a less common site for infection in early childhood but still an important consideration in certain clinical scenarios.

## Correct Answer: C.