Explain what is happening to the Ghost of Christmas Present as time passes?

As the evening passes, the Spirit of Christmas Present grows older and older. He only lives for one night, a night during which he brings happiness to the sick, the poor, and the struggling.

It was a long night, if it were only a night; but Scrooge had his doubts of this, because the Christmas Holidays appeared to be condensed into the space of time they passed together. It was strange, too, that while Scrooge remained unaltered in his outward form, the Ghost grew older, clearly older. Scrooge had observed this change, but never spoke of it, until they left a children's Twelfth Night party, when, looking at the Spirit as they stood together in an open place, he noticed that its hair was grey.