reflects the timeless nature of the ghost, and could be a metaphor for scrooges past. it knows his childhood and him as an old man, and is prepared to show scrooge his past, which frightens scrooge (we know this since he attempts to put a cap on the ghosts light). a child has connotations of innocence and purity. we can interpret this as scrooge being as such as a child. in contrast, he is now an old man, broken and emotive.
alternatively, this ghost could be a personification for memory. this is supported by the quote "fluctuated in its distinctiveness", just like how memories fade and alter in the subconcious