Determination of any child's ideal weight depends on several factors, including most importantly the child's height. Plotting a child's weight on a growth chart, which every pediatrician does, will help determine if the child's weight is appropriate for the height. Girls on average weigh about 99 pounds at 13 years of age, but the variation can range from about 78 pounds (5th percentile) to 134 pounds (90th percentile). A girl weighing 134 pounds is normal weight if her height is also around the 95th percentile (about 5'6" tall). The weight and height can be used to calculate body mass index (BMI), now used to determine appropriateness of any child's weight. As you can see, the BMI relates weight to height. BMI calculators can be found on line (see link below). BMI curves also should be plotted in the doctor's office, so for example a girl who is 144 pounds and 5'6" tall has a BMI of 21.6 which is the 79th percentile BMI. If the BMI is greater than the 85th percentile, the child is considered overweight. Further discussion about the BMI and normal weight determination is also found in the second web site below.