Weight goes by height, not age. 103 is definitely on the low end. If you are four feet ten inches tall or less, that is normal. If you are taller than that, you are under a healthy weight. It is more important to eat healthy and be physically fit that to be light. Muscle weighs more than fat, so when you build a better body with muscle you will gain weight, but look better. I am sure you are beautiful.Here is chart: http://www.healthchecksystems.com/heightweightchart.htm
There is an app called teenBMI that lets you put in your age, height, & weight, then tells you BMI & if you are overweight, normal, or underweight. Lower weight for height can be normal depending on age, as many teens are in the midst of growth spurts that don't always keep up with weight gain.