Why do you think the old man is included in the story?

He's actually not an old man rather a symbol for death. IHe is a big part of his moral. Check this out,

http://www.gradesaver.com/the-canterbury-tales/q-and-a/if-the-old-man-in-the-pardoners-tale-wants-to-die-and-looks-like-he-is-already-dead-why-is-he-not-allowed-to-die-52183/