Hi all. I'm not sure if I'm skinny.. I'm 13 years of age,5'11 and I weigh around 140 pounds... I have small shoulders and I want to know good exercises for those. Also what exercises would stunt my growth or weaken my bones or something as I'm only 13.. and is it ok to work with 5kg dumbells to gain muscle.. or should I use something heavier and more difficult. Thanks in advance.