Man, really good points here. I know there is a rush to mature and go "adult" these days, but my body changed so much from the time I graduated high scool until the time I turned 22. Granted I stopped playing sports at 16. If you are serious about your diet and getting in shape, I would take wrestling in high school to suppliment your MMA. Also, meal replacement, or weight gainer protein, and whey protein are really the only things you should worry about in the muscle shops.
Dont take creatine, or any type of supps until your are 17.5. Then dont waste your time with Shotgun, thats garbage. Go for the BSN No-Xplode (They just came out with the NT version which is much better on the stomach). It used to be cheaper until the UFC started sponsoring it. Still the best stuff on the market. Ive tried them all. My buddy owns a supp store. Keep practicing. Eat right, train hard. The muscle and size will come. Good luck.