Put items in a book, article or essay that provide ways to tell the date, year or age of the writer. This is called "dating" the item.
Decide whether or not you want to leave your autobiography undated. Sometimes it is better to leave readers in suspense so they don't feel cheated out of an ending. If so, you will need to use generic terms. Instead of "The year I turned seventy," you could use "The beginning of my "golden years."
Decide when you want to end your autobiography if you want to end with a date. There are many ways to do this. Remember that whatever date you choose, there are no right or wrong times to end your autobiography. This is your life and your work of art.
Decide what the most significant age in your life was. This may be an end date for your first autobiography. You may choose to create a "sequel" this many years after the first one.
Decide if you want to write your autobiography ending at the age you became an adult or, maybe, the year you finish writing it.
Write a second book if you end your book at your adulthood or a younger age than seventy. You may even wish to write more than that.
Keep the sequel undated if you decided to leave the ending of the first book undated.
Another approach would be to date the beginning of the second book to clear up the mystery of when the first book ended.
Enlist help from family, friends or children if you are taking the approach of multiple sequels every so many years. If something should happen while you are in the middle of a sequel, have someone you can trust to finish your sequel. Inform them to end it with information about your death.