It depends on your height. The weights of 15 year old girls vary depending on their height which is used to calculate their ideal body mass index. Consider calculating your BMI to know if you have the ideal weight for your age. You can use the website http://pediatrics.about.com/cs/usefultools/l/bl_bmi_calc.htm for more information.
It depends on your weight and height and body type. Also your weight fluctuates all the time throughout the day and nite, so to get the most accurate weight, when you weigh yourself be sure that you have not eaten in quite a while (like when you wake up in the morning), be sure that you have digested all food and drink, and it's best to weigh yourself with no clothes on because believe it or not, clothes can actually add about 1-2 lbs of weight.