Wonderful clarification from Katie Shea of Caren Johnson Literary Agency on what memoir is not: a story of your life (that would be an autobiography). Here's her answer regarding memoir manuscripts from the May 26 interview at Guide to Literary Agents:
GLA: Where are people going wrong in their memoir submissions to you?
KS: One of the worst things I see with memoir is when the writer starts from the beginning of their life to where they are now. Memoir should be only a chapter of your life. I have been pitched memoirs that could be divided into three books!
For memoir writers, choose your strongest or favorite theme and then work from exactly when it started to exactly when it ended. Do not include the before and after. I do not want to be reading a book from when you were two years old up until you are 43. It just doesn't work.