Age doesn't matter to publishers. They are looking for good writing. If you land a book contract, your parents have to sign it for you, is all.
However mature you are, you need to clean up the grammar and punctuation to bring it up to a professional level or they'll reject it outright. Your manuscript is going out on a "job interview." It needs to be polished up and shiny!
Strunk and White's ELEMENTS OF STYLE is all you need. All writers have that book and use it.
FIRST--finish your book.
NEXT- Get feedback from other writers.
This site is a SAFE writer's board run by pros. It is free and they have a password protected Share Your Work forum. They can also help you stay clear of the sharks in the publishing pool.
Feedback, beta readers, polished, and re-polished, a book has to be the best you can make it before you send it out. I thought my first was ready, but two years of rejections and rewrites taught me better.
(It did sell, I landed a multi-book contract.)
How to find a publisher:
Go to a bookstore. Find books like yours. Look inside the front pages to get the publishers' websites. Follow their submission guidelines to the letter.
Look up websites of writers with work similar to your own. See if they mention who reps for them. A short polite mail asking if they can recommend an agent is not out of place. Do not tell them about your book, that's seen as poor manners.
Obey YOG'S LAW: "Money flows TOWARD the writer."
This means NEVER pay to get published or to enter contests or pay a fee-charging agent. NEVER. Obey Yog's Law and you won't get ripped off.
Go to the 808 section of your library which has all the books on writing. They will help you steer clear of the rips.
Writer's Market 2010 will be in the reference section. It lists all the commercial publishers and what they're looking for.
But you can't be in a hurry, or assume you'll get paid a fortune. The Brenda Hiatt link below will give you a general idea about book advances. Always assume you will get the lowest number. You'll be the newbie on the block. It takes time to work up to getting the real money.
STAY AWAY FROM PUBLISHAMERICA. They are the BIGGEST ripoff site of them all. They say they're free and don't want your money, but that is a LIE. You end up buying overpriced copies of your own book!
They will not put your book into a store--but lie and say that you are supposed to do that. Most writers submit their book, and whether it is good or horrible, it's quickly "accepted," (they take everything!) and then you never hear from the poor writer again. Your rights are tied up for seven years!
Just google "publish America" + "scam" and find all the writers who got ripped off by them. They are what is called a "print mill." you don't want anything to do with them. They ruin lives and careers.
If any publisher or agent advertises on Google, it will only lead to a scammer or a vanity house. A real publisher or agent doesn't advertise!
Again--the 808 books at the library will help you on all this!
Helpful sites for writers:
Why manuscripts are rejected: http://nielsenhayden.com/makinglight/archives/004641.html
And just because you've finished a book doesn't mean it's commercially viable. Pro writers finish a book and start writing another. It keeps your head from exploding while you're waiting to hear back from agents and editors!
See you on Absolute Write! ;>)