When a child is bourn it is in a world it does not know. It knows not how to walk, eat, see, hear, etc. Therefore it is taking everything in. Every nuance around him is new.
An adult see's a tree. To him it is a tree. To the child it is new, as is every plant, animal, car, you get the idea. Perhaps time goes by faster because we have taken more in and so are less aware?