The retrospective narration helps to underscore the theme of "what is lasting" which is central to the story. Finny changes as does Gene, but certain memories last; certain pains. In recalling this story from a future point in time, the narrator can explore ideas of how formative his school days actually were as well as exploring ideas of what did not change and what he (Gene) discovered about himself during those days. 

You need to be aware of the narration and how it affects the story. The mode of narration for this tale is called retrospective first person narration, in that we have an older, wiser character reflecting back on their actions, thoughts and speech. However, as always with a first person narrator, you need to consider how "reliable" or not the narrator is. A first person narration is necessarily a limited point of view, as we do not have access to the thoughts and motivations of other characters - only how the narrator perceives and understands them. Likewise, whilst we have the benefits of added time to reflect and process the action, we have the negatives of time which can change or alter memories.

In terms of strengths, an older narrator is better able to be more reflective and analytical. The problem lies in the fact that the events of the story are not as easily remembered fifteen years down the road than when they happened.  The story might be a bit more realistic if seen through the eyes of 15 year old Gene. Also, an older narrator tends to sum up what was all "meant" by the experiences, whereas a younger narrator simply lives it.