Final answer:
Sphenoid sinuses generally start becoming noticeable at the age of 5 years old, consistent with the overall developmental changes happening in the skull during early childhood.
Step-by-step explanation:
The sphenoid sinuses are typically present at the age of 5 years old. During early childhood, various changes in the skull's structure occur, including the enlargement of the paranasal sinuses. These developmental changes contribute to the transformation of facial structures as they grow and adapt to accommodate newly erupting teeth and other anatomical developments.