Muscle size building?Seek professional advices.?

hello.i'm 16 here.i had joined the gym for a year.though there are some changes, but it is just too minimal i guess? i had been watching those korean stars, they are only 20+ and after a half year, they came out with really big and well-defined muscular bodies which the instructors in my gym have.i mean my instructors are really strong and they lift heavy weights.i mean , how could those 20++ people transform and being so muscular with just a half year plus while i had been going the gym 3 times per week for whole year and my result is still unsatisfying.has it something to do with my genetics and age?