Age doesn't matter. Publishers just want a solid idea! For example Nancy Yi Fan published her novel Swordbird when she was 12. Hannah Moskowitz published Break at 17. There are many other tween authors emerging!
Teen Ink is a great place for young writers to start; they only publish work by teens. (http://www.teenink.com/)
Teen Voices publishes student work also; they do not discriminate because of your young age and exists as an online and print magazine for and by teen girls. (http://www.teenvoices.com/)
There is a brand new imprint which was just launched by Medallion. They will publish YA books only by authors between the ages of 13-18. Medallion is currently seeking submissions for its new Ya-Ya line; submission guidelines are posted on the company’s website.
If you are looking to get published and you have edited your work and have it in the best shape it can be, start a blog! They are free and you can post a tiny synopsis of your work. Keep it short, you don't want to give your idea away to others. Jessica's agent found her that way. The agent randomly Googled something like "tween horse authors" and found Jessica's blog and contacted her. It hasn't been the typical way that agents have found authors, but it has happened.