I have no idea what the average weight is for my 13 year old son, but I'm sure it is less than he weighs. He is about 160 pounds, but he is really solid and muscular. His bodyfat is in the normal range and when he flexes, his muscles are about 3 times the size of mine, a regular weight lifter.
He had a belly for the past couple of years and asked me how much he should weigh. I thought he should lose about 10 pounds, but I didn't tell him that because he is continually growing, and I didn't want him to try and lose weight and grow at the same time. I told him to not focus on the scale, and just eat healthy and exercise and it would all work out. He reads labels now and says no to junk, drinks lots of water and exercises so now he looks 'just right'.