One definition of childhood obesity is having weight in the 95th percentile or above. According to these charts, that would mean a BMI in the 24-25 range for a boy, and in the 25-26 range for a girl. At your height, this translates to
1.422*{24, 25, 26} = {48.4 kg, 50.4 kg, 52.4 kg}
If you are a 12-year old boy, you would be considered obese at 50.4 kg. If you are a girl, you would be considered obese at 52.4 kg.
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The range above is taken from the 95% curve between 12 yr old and 13 yr old. The closer to 13 you are, the closer to the upper end of the range you can be. You would be in the 5th percentile or so for weight at about 0.6 times the above values. In other words, 90% of all 12 year old boys and girls were between 29 and 52.4 kg at the time the statistics were accumulated. Note that the charts are published by the National Centers for Health Statistics in the United States.
1.422*{24, 25, 26} = {48.4 kg, 50.4 kg, 52.4 kg}
If you are a 12-year old boy, you would be considered obese at 50.4 kg. If you are a girl, you would be considered obese at 52.4 kg.
_____
The range above is taken from the 95% curve between 12 yr old and 13 yr old. The closer to 13 you are, the closer to the upper end of the range you can be. You would be in the 5th percentile or so for weight at about 0.6 times the above values. In other words, 90% of all 12 year old boys and girls were between 29 and 52.4 kg at the time the statistics were accumulated. Note that the charts are published by the National Centers for Health Statistics in the United States.