What age of person does the First Ghost look like in A Christmas Carol?

The first ghost looks both old and young at the same time.

Fitting the changing nature of the past, where memories are murky, the Ghost of Christmas Past has an interesting look.  It is described by Dickens as both old and young at the same time, and both man and woman.  The ghost is everyone.

It was a strange figure—like a child: yet not so like a child as like an old man, viewed through some supernatural medium, which gave him the appearance of having receded from the view, and being diminished to a child's proportions. (Stave 2)

Waiting for the ghost, Scrooge is nervous as a result of Jacob Marley’s prediction that three ghosts will visit him.  He does not know what to expect, and when the first appointed hour comes, he is exceedingly nervous.  The first ghost appears to him through his bed curtains, and has an odd appearance almost as if it is glowing.  The description goes on to explain how the ghost’s physical features are contradictory.

Its hair … was white as if with age; and yet the face had not a wrinkle in it, and the tenderest bloom was on the skin. The arms were very long and muscular; the hands the same…. Its legs and feet, most delicately formed, were, like those upper members, bare. (Stave 2)

The ghost glows almost like a candle, and is wearing white.  It’s varying and contradictory appearance reflects our view of and memory of the past.  As the ghost takes Scrooge through his journey, he shows him the events in his past that have most made him the man he is in the present.  Some are pleasant, and some are almost unbearable.

Scrooge’s second ghost, The Ghost of Christmas Past, makes a big impression on him.  He was already baffled by Marley’s shackles, and now he is confused by the ghost’s inconsistent features.  Yet the ghost shows him things he needs to see but does not want to, so eventually Scrooge can take no more and puts out his light.  It will do little good, for the next ghost is coming!  By the time Scrooge meets with the Ghost of Christmas present, he is already reflective.