The main purpose of the stanza is to show how the boy has changed. The narrator describes his memories as a boy, when he used to go to the swimming hole. He has beautiful memories, and it makes him happy to remember them. However, he then says that time has taken his toll, and that the boy is now an old man. The stanza, therefore, describes how the passage of time has changed the boy.
Well, the stanza does describe the swimming hole and the speaker as a boy. But I believe the point of the passage is to display how the boy has changed.
