Why is starting to study medicine content before med school such a bad thing?

I've heard a look of people say "pre study wont help at all for med school and it's a bad idea". I get that but as someone who is 15 and in my summer holidays for 6 weeks, I don't understand why getting textbooks and watching lectures on Anatomy, Physiology, Bio-Chemistry etc will hinder my progression at all. I'm planning to go into medicine because of my love for these subjects. I've been itching to start learning these topics and have fundamental understanding in them already. Why should I have to wait around doing nothing useful with my life until I graduate or if I even get into med school to learn.