What is considered underweight for a 16 year old female whos 52.5?

For a female measuring 5ft 2.5 inches, a weight of under 103lbs is considered underweight. Normal range would be from 103 to 136lbs. It is important to understand that at 18, your weight may normally be lower than when you get older because of metabolic rates. If you are underweight, try changing your diet to incorporate more healthy fats and lean calorie dense meats, fruits and vegetables.