An autobiography is always written in the first person, and it narrates the whole life story of the author, from start to the present in the person's life. It is an account of all the places, people and events that made the writer's life what it has been, as well as the lessons learned and experience gained.
Narrative is, essentially, telling a story. In the case of an autobiographical narrative, the story is focusing on one or a few main events that shaped the author's beliefs and personality and changed his way of thinking. The main feature of an autobiographical narrative is that it has a specific plot, instead of being a list of everything that the author experienced. In the narrative something happens, which hooks the reader's attention and keeps her reading from then on, anxious to see what will happen next.
An autobiography might be a list of people, places and events, but this account is not necessarily given in a chronological order. The author often chooses to start with a striking event and then go back to take things from their actual beginning, after he has ensured the reader's full attention. The narrative, however, always tells what happened chronologically, since dealing with a specific time of a person's life cannot accept switching from one time to the other too often, or the story might confuse its readers.
Time of Writing
Even though the author can decide to write her autobiographical narrative at any given moment, and usually does after an event that marked her as a person, an autobiography is most often written nearer the end of a person's life. To give the account of a life story, you need to have the events that compile this life, and this is usually after the writer has experienced a lot of things. However, a life-changing event or the decision to look back and reflect on a past experience, can come at any time, so the narrative can be written even at a young age.