**Core Concept**
The development of paranasal sinuses is a complex process influenced by genetic and hormonal factors. The sinuses are not fully developed at birth and continue to expand and mature throughout childhood and adolescence.

**Why the Correct Answer is Right**
The sinuses that are not present at birth are the frontal sinuses. The frontal sinuses typically start to develop around 2-3 years of age and are usually fully developed by the age of 6-7 years. This delay in development is due to the fact that the frontal sinuses are derived from the frontal bones, which are not fully ossified at birth.

**Why Each Wrong Option is Incorrect**
**Option A:** Maxillary sinuses are present at birth and are one of the first sinuses to develop. They are formed from the maxillary process, which is a part of the first pharyngeal arch.
**Option B:** Ethmoid sinuses are also present at birth and are formed from the ethmoid bone, which is a part of the nasal cavity. They are composed of multiple small air cells that are connected to the nasal passages.
**Option C:** Sphenoid sinuses are present at birth but are not fully developed until around 2-4 years of age. They are formed from the sphenoid bone, which is a part of the skull base.

**Clinical Pearl / High-Yield Fact**
It is essential for pediatricians and primary care physicians to be aware of the normal developmental timeline of the paranasal sinuses to distinguish between normal variation and pathological conditions.

**Correct Answer: C. Sphenoid sinuses are present at birth but are not fully developed until around 2-4 years of age.**