I don't know if this applies to you but the other mistake that many make is to spend too much time in the gym. Many young people trying to get big are very determined and as such are their own worst enemy in that they do tons of reps and tons of sets. To gain mass particularly with a fast metabolism concentrate on heavy excersizes and compound movements. Use a wieght that will cause failure at 8 reps and limit your sets. Another mistake young guys make, do not forget legs (leg press and squat particularly) legwork is essential at your age to force growth to the rest of your body if you want big arms believe me leg work will get them as quick as curls. . Go to the diet section and post up your diet and you will probably find you have plenty of room for improvement. Focus on eating 7 solid meals a day and limit your whey to a PWO drink for your other meals get your protien from eggs (breakfast) then fish, chicken, beef (for a hard gainer this can be hamburger to keep the costs down)