You can not calculate a healthy weight by age and average as everybody is genetically differant. The best way to check the health of the twelve year old is. BMI, (body mass index), which is calculated by either:- Height and weight. OR Weight and waist size.
I am not certain, But i think that is pretty bad! The average weight for a twelve year old would be around 120 lb to 130 lb. I am a twelve year old female and i weigh 140 lb, And that isn't very good either.. ;p
There is no average "perfect weight" for children. The best way to calculate whether or not a twelve year old is a healthy weight would be to calculate their Body Mass Index (BMI) which uses both weight and height to give a ratio of body fat.
in my point of view who cares what your weight its all about your self image of your self if you say your fat you will be fat if you say i am beautiful you will be beautiful and all that you should care about is how you look in the mirror!
95-110 But remember if you are over or under this limit, it all depends on your height, bone structure, and genes. It doesn't mean that 95-110 is the ideal weight for a twelve year old, its just the most common I guess.