A 13 year old boy should weigh at least 75 to 80 lbs which is a healthy weight.
A 13 year old girl usually weighs between 76 lbs to 80 lbs which is average healthy weights for 13 year old boys and 13 year old girls.
However some 13 year old boys may weigh as much as 145 lbs and some 13 year old girls may weigh as much as 146 lbs.
But at those higher weights they are considered overweight.
If the 13 year old is weighing under 75 lbs then they may be underweight and should see a doctor.
But all people are different and some people may weight less and some people may weigh a bit more than others.