This is my first time trying to write a book, and I usually just plow through stories so fast that it ends up being like 50 pages. I'm at 73 pages right now, wordcount being a bit over 10,000. It's probably going to be around 150 pages total, as I find too many subplots boring. Is this too short?
Too short is relative. For a novel, probably. For a novella, perhaps. For a short story, no. For flash fiction, too long.
Don't worry about length. Worry about whether you have told the story in the number of words needed. I started writing a short story; it turned into four (short) novels. The story just kept developing. However, I wanted to write another novel and the idea I had was only a short story.
Write and revise. That way you become a good writer.
I wrote several stories at your age I thought were good. However, they weren't novel length. Writing, not length, is what is important.