One of the things the reader learns about Ethan from another character is his age in the prologue.
Ethan is introduced to us in the prologue as a middle-aged man. His age is very important, because he looks much older than he is. His actual age is learned from Harman Gow.
There was something bleak and unapproachable in his face, and he was so stiffened and grizzled that I took him for an old man and was surprised to hear that he was not more than fifty-two. (Prologue)
The significance of the prologue is that it introduces us to Ethan after the story. We know that this story will change him. He will be banged up in a terrible accident, he will look older than his age, and everyone will talk about him.
By beginning the story this way, several important elements are established. We can tell that Ethan is important to the story. We can tell that Ethan is the topic of conversation. These will be significant as the book begins and we find out why. It creates suspense, because the reader wants to know what happened and what made him that way.