13...you kidding...like what G said how good you want to become is up to you. Our junior instructor is 17/18 and he spars with me. Extremely talented...I'm bigger...got longer reach, but he's got wicked combos and is a future fighter should he choose to. Only thing is he doesn't really train BJJ for some reason... Anyways I've seen some pretty talented kids in the gym. As the sport grows you'll get a leg up for training that young. It aint easy I'll tell you that much. I learned that training is the equivalent of working. Once you hit the gym you'll know what I'm talking about.
First it's Muay Thai, then BJJ, wrestling, grapping, and each has their own components. On top of that you gotta weight train, core excercise, conditioning (I got good tips from G), and dieting. That's when I realized I was just training for the love of it. To become pro you gotta put in work just like everything else in life.