I know it sucks, im almost 22 and I still weigh 130 pounds. All i can say is if you want to gain weight you have to go to the gym, there is pretty much no other way. Eating junk food doesnt do anything except make you unhealthy (you can still be skinny and have high blood pressure etc.) I got up to 145 when I was working out but now im back to lithe old me.

