Why did I gain weight so much?

I've related my story in SHINE Yahoo and it was about me gaining weight. I use to be really skinny in high school, 5'5 at 45 kgs only. Right now I'm 5'6 at 60 kgs. I pushed myself to put on weight when I was young cause my metabolism was so high, no matter what I ate... I didn't gain weight. (I looked really ugly when I was skinny!) After I changed my diet to gain weight, I realize that it's harder to lose weight now than before even though my metabolism was really high. Is it possible that my metabolism had slowed down due to aging?