I must have miss this option along the way setting upon my first book. Now how do I go back and add it or change it? How do I do this upfront before its published?
If you go onto your bookshelf and to the square grey box to the right of your book, select Edit e-book details. This brings up the first page of the publishing process where you entered your book's details originally. The section on setting age ranges is towards the bottom of the page. You should have completed it when you set up your book, but it's not too late to do it now.
Once you update it, continue by saving until you get to page three where you need to click on publish for the changes to go live.