Calculate the correct amount of protein. According to Drugs.com, 12- to 14-year-olds should consume approximately 1 gram of protein per kilogram of body weight each day. Girls aged 15 to 18 need slightly less protein, or 0.9 grams per kilogram of body weight. Find out how many kg your teenager weighs by dividing her weight in pounds by 2.2. For example, if your 13-year-old weighs 100 pounds, she weighs 45.5 kilograms, and should consume about 45 grams of protein each day.