Telling someone that they are skinny can be as hurtful as telling an overweight person that they are fat. I was a very skinny child too. In fact when I was in high school I had to shop in the little girl's dept. and I was horrified by it. I didn't start to gain weight until I was 35 and now I'm trying to lose it. When people tell you that your skinny you should let them know that it hurts your feelings. What are some of the things you have tried? Some young people just have a quick metabolism and it makes it difficult to gain weight but it does change as they get older.