By this point, they had almost reached their full adult height of five feet, and their wings measured more than seven feet when outstretched. Their fluffy down had long been replaced by mottled tan-and-white feathers, which were growing whiter by the day, though their heads had yet to develop the signature crimson cap.
(11) This is just describing changes in their physical characteristics, but the implication here is that 5 and 33 are maturing. Qualities of people I know as they've matured come to mind here.