When you'll be 20 years old your body no longer be so active , it's about metabolism , and digest than in your stomach metabolism decrease , we all have different kind of body , but as you notice kids growing and changing body every year their metabolism so high so in that time for some that's why hard to gain fat they will never be fat that's why me when I was younger I was so slim like a stick it's because of metabolism but now my body changed metabolism decreased and I can get fat more easily , but I not gonna be fat because I workout