I don’t see a problem with a 15 year old taking creatine. Make sure you know what you are doing. Drink plenty of water and don’t abuse it.
I started taking creatine in eighth grade when I was 14 years old so I don’t see a problem with you taking it. But don’t think that it will magically make you stronger. It helps you by giving you a greater ability to train. It doesn’t help you on its own. With out the extra training that it will allow you to do, you won’t have greater gains than you would have without taking it.
I noticed immediate gains when I started taking it. My bench went up 10 pounds in a week. I feel like creatine has allowed me to make more consistent progress over the years. I used it all of my ninth grade year when I was 15 and my bench went from 185 to 265. I’m not saying that was all because of creatine, but it definitely helped…I took it all year.