Gluttony is eating food for the sake of enjoying the taste only. The rich Romans used to feast, and then throw up, so they could eat some more. At your age, it's pretty hard to eat too much, as you are still growing, and need more food than you might think. Now, if you start popping out of your pants (but not your shirt), and your doctor tells you something is wrong with your weight but can't find a medical cause, look for a psychological cause. The sin of just eating because you enjoy it is probably the last thing that it will be.