The youthful face of the Ghost of Christmas Past symbolises the power of memory. Therefore, when the Ghost is with him Scrooge is able to recall his whole childhood and youth as though he had never forgotten it. Indeed, the visit of the Ghost shows that he has not really forgotten it; rather, he has kept it all locked away in his heart.
Remembrance of the past proves to be the most healing power that Scrooge experiences. In an instant his old crusty, miserable exterior falls away from him as old sights and sounds surround him once more. This is the beginning of his transformation, his redemption, as after so long he once again opens up his heart to feelings and emotions and the animatedness of life. It is the remembrance of his innocent child-self that sets him on this road.
The Ghost of Christmas Past is of rather strange appearance. Although it has such a youthful face, without a 'wrinkle', and the softest skin, its hair is white; and although it is small, like a child, it has the aspect of an old man. Moreover, its appearance is always shifting:
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 (Stave II)
The Ghost, then, cuts a curious figure, with its outline, its shape, constantly changing, fading away, re-appearing. This mimics the elusive, shifting quality of memory itself.