Self publishing is possible at any time and at any age. All you need is a word processing programme, laser printer and a way to bind the finished documents. Make fifty copies and advertise your publication to see how it is received. If that works, print another fifty and keep going. To go anywhere near a Publisher is EXPENSIVE and that would need parental permission. Prove the acceptance of your book as a commercial proposition first in this way. ALSO: open up all the self-publishing links showing around this page and they will spell out the ways available to you.
Christopher Paolini was a best selling author at 15 years old. You have to prepare a manuscript and send it out to different publishing companies. Once you have parental consent, and a publisher, you are an author.