It  is never too late to get started on your memoir.  When you are ready,  just start writing.  You can begin with a person, place or event that  had special significance in your life.  Or, think of a period that was  life changing for you.  In any case write about the impact on your life  and those around you.  It may help to start with a journal where you record information about your life.  Then, let these memories become the basis for the different chapters of your memoir.  It can be a lot of fun and very fulfilling to get your life on paper for others to share.