Stop thinking.
Write write write. Use your gift and follow what you've got inside you.
Do it. Don't wait. Never wait.
People say that a young writer's inexperience shows in their writing
There's a lot to be said for approaching something with the Beginner's Mind. Many artists spend their careers trying to get back to that state.
I've had millions of ideas for a book and most of them have either been abandoned or crumpled and thrown into a trashcan.
Yeah. That's called "being a writer." That'll probably never change. Keep going.
Don't worry about who you are, just write your fucking heart out. Your age/race/sex/etc. do not matter. People will always judge you for an endless, ever-changing list of reasons. Forget 'em. Don't even bring up your age, especially you don't have to directly interact with the publishers. Your work should be your focus, and that's all. You aren't a "young writer," or a "gay/straight writer," or a "male/female writer," or a "black/brown/white writer," you are a writer.
I don't know whether I could get past the age barrier.
What age barrier? If you hadn't just told me your age, I'd have no idea. Forget about it. Plenty of experienced adults produce cute fantasy books and disdainful romantic cheese. If you suck as a writer, stop sucking. You are in the enviable position of having lots of time to learn to not suck at writing.
a prejudice which has been my major drawback when it came to my dreams for publishing a book at this age.
It's nearly damned impossible for anyone to get a book published at any age. Listen to what editors and publishers say about your work. Do they have criticisms of the material? If so, read it and think about it. Maybe your writing needs work, and still would if you were 35. Do most young writers suck? Yes. And most of the older ones, too. Don't worry about publishing. Worry about WRITING.
If your real concern is, "Do I keep writing while my studies are suffering?" then you need to adjust your thinking a little. Your studies are suffering because you aren't putting in the time and work. School is YOUR JOB right now. You only get one pass at it, and I can confidently say that you DO NOT WANT to screw that up. If you want to write, you'll need to do it after your job is done. Plenty of writers write while working full time jobs. Do not use writing or your talent for it as an excuse not to do your job.
Whether or not you choose to work hard on your studies will likely have a profound impact on the rest of your life--including your writing. Learning even seemingly unrelated things can help inform and improve your writing in ways you can't imagine yet. Read and learn voraciously. The world is a very big, very weird place. Very few of us really grasp how true that is.
Stop telling me how 14 you are. I don't care. I love to read good writing, though, so hit me. Post a story. Here is a place to find subreddits for many kinds of writing. Find what you do. Post. Find a decent editor. (maybe an English teacher? Maybe someone on reddit?) Submit stuff to magazines, online and off.