As the Lady of the Lake wanes in power throughout the story, with each sighting we see her "necklace of fish" reducing in number. Similarly, Queen Mab's hair grows grey as her power wanes. Merlin, being a mortal who contains magic within him, only wanes in appearance with the passage of time.
According to Sam Neill's autobiography, Sir John Gielgud was 94 years old at the time of filming and performed his cameo in one day. When the director asked him why he was still working and not retired, Gielgud jokingly responded, "I have a very expensive boyfriend."
Because of his strong performance as Merlin in this film, Sam Neill was offered the role of Gandalf in The Lord of the Rings: The Fellowship of the Ring (2001) but turned it down due to complications with filming Jurassic Park III (2001).