What is the ideal weight for a 16 year old girl who is 5'7?

According to the website "Lifestyle" at 16-17 years of age the average height for a boy is 67 to 70 inches. You are at the lower end of their average height range but you are also 16 not 17. The same website lists 130-150lbs as the average weight for your height range so I would image that as your height is at the lower end of their average heights then your ideal weight would be close to the lower end of their range, therefore around 130 pounds (9 Stone 4 Pounds).