All are of adult size at bih except:

Maxillary antrum "The tympanic cavity and mastoid antrum , auditory ossicles and structures of the internal ear are all almost fully developed at bih and subsequently alter little." -Gray's Anatomy 40/e p623Maxillary antrum reaches adult size by 15 yrs of age. All paranasal sinuses except maxillary (and sometimes ethmoidal) are rudimentary or absent at bih. They enlarge appreciably during the eruption of the permanent teeth and after pubey, events that significantly alter the size and shape of the face.Some students were of the opinion that in the present question 'orbit' was the option instead of maxillary antrum. In that case the answer would be orbit. "Orbit reaches its full adult size by 15 to 16 yrs of age." Also Know:Though the mastoid antrum is well developed at bih, the mastoid air cells are not developed and are merely minute antral diveicula at this stage. The mastoid process develops in the 2"d yr and is invaded by air cells in the 6ill yr.(Ref: Grays 40/e p417)