I don't get it I'm 18 and never exercise. I eat breakfast snack dinner snack tea snack everyday and I only weigh 6st 2ibs. I went to my doc and he sed I just had a vv high metabolism but my teachers are poking ther nose in and asking me if I'm eating properly. I say yes and even eat in class when allowed. Most of my mates are quite big size 14 16 18 and a size 20 girl.just a few. I am not even a size 6!!! My clothes come from the kids section of new look or miss selfridge!! I love being thin but my mates are all on diets to get down to my size.
Your probably naturally thin, if not then just have a balanced diet, drink lots of water and try to get some healthier habits. And maybe build up more muscles, it would make you weigh more and be healthier and capable of more stuff.