I am writing a novel called "Kimberly Rose," but the thing is, I'm 12. I'm not sure if there is an age cap when it comes to publishing books. Please tell me if you know.
First of all, good for you! Secondly, have a look at this: Children as Authors.
I don't think your age will affect the publishing of your book, but the quality of your writing will — just as it would any adult author. I suggest you have a proofreader read your story so you can know if it needs adjustments or corrections.
Good luck! I really hope your book gets published and you write many more in the future!
Your age shouldn't have an affect on whether or not you get published. So You Want to Be a Teenage Author? Published Teen Authors Tell It Like It Is has lots of case studies on writers who got published as teenagers, and how it effected their writing careers and lives. The general advice is not to tell an agent or publishing house your age until you meet in person, this prevents your query from being thrown out the window immediately, as many people believe young people cannot write with the depth an adult can, simply because they haven't written enough.
It's fantastic you're already writing a novel at 12, so make sure to keep at it. Have friends, family or teachers read a draft for feedback and use this to make your writing better. Once you've finished that all important first draft, feel free to pat yourself on the back, it's hard finishing a draft and not giving up partway. Once the celebration is over, it's time to think about editing. Your first draft won't be perfect, so put it down for a month, maybe write something else for a bit, before coming back to look at your novel with fresh eyes, ready for editing!
You're still young, so you have plenty of time to perfect your novel and your writing, don't rush something out of a lower quality than what you would be happy with seeing under your name just so you were published young. Just remember to always be proud of what you've done. Even thinking as far ahead as publishing is a great ambition that I certainly didn't have at 12!
At your age you shouldn't try to judge yourself by commercial success, but by whether you finished the project successfully. That includes not only coming up with and executing the idea, but also maintaining quality control over everything.
Try to keep an eye out for markets which appeal to people your age. (I talked recently to a 16 year old who published and sold her first comic book 2 years ago. Now she is running her own comic book company). I once created a 75 page Dungeons and Dragons module at the age of 14 -- I was very proud of that -- I ended up enjoying it at a well-attended gaming convention which I organized at 15.
I recently judged a fiction writing contest for 7th and 8th graders. Although many works dealt with typical zombie and superhero plots -- and some of the writing was horrible -- generally their ideas and characters and dialogue were interesting and original.
The only thing I'd warn you against is trying to finish things too quickly. Write it, let it sit around for a while, and then come back to it....
Finally, we are in an age where publishing is relatively easy; the hardest part is being to package and market what you've written.