What age of a person does the first ghost looks like in A Christmas Carol?

The Ghost of Christmas Past looks both old and young at the same time. 

On Christmas Eve, Scrooge’s friend Jacob Marley visits him.  Normally, that would not be a big deal.  However, Jacob died seven years before!  He visits Scrooge as a ghost.  Marley tells his friend that Scrooge is wasting away his life focusing on money, so he has arranged a special opportunity for him.  Three ghosts will visit Scrooge on three different nights.

Scrooge wakes up to hear the clock chiming.  The next thing he knows, he is faced with the first of the three ghosts.  This is a baffling figure.  Somehow, the ghost appears both old and young at the same time.

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)

The ghost is also glowing.  It has white hair, but youthful skin.  The contradictions continue.  The ghost has muscular arms, but delicate feet.  It carries holly and is dressed in summer flowers.  It seems like whatever else this ghost is, it also is the opposite.

Perhaps most bizarrely, the ghost flickers like a candle, so that you can’t see all of it at once all of the time.

[So] the figure itself fluctuated in its distinctness: being now a thing with one arm, now with one leg, now with twenty legs, now a pair of legs without a head, now a head without a body: of which dissolving parts, no outline would be visible in the dense gloom wherein they melted away. (Stave 2)

The ghost’s voice is “soft and gentle” and when he tells Scrooge that he is the Ghost of Christmas Past, Scrooge asks him if that means “long past.”  He tells Scrooge that he is the ghost of Scrooge’s past.  The ghost shows him his childhood and early adulthood, and it has a great effect on Scrooge.  He begins to soften.

The ghost’s strange contradictory looks are symbolic.  The past is never clear to us as we look back.  We are seeing memories, and like a dream, sometimes things fade in and out.  The ghost’s indistinct nature mirrors the elusiveness of the past.