Is 140lbs overweight for a 13 year old?

Use a BMI (Body mass index) to determine that. According to the BMI you are in the normal category, so no you are not overweight. However, you are at risk of becoming overweight.

I will have to correct that. You ar a healthy weight and the only reason that are at the risk of becoming overweight, is of you sit on the couch all day and eat junk. Let your body do what it's supposed to do, eat healthy, get active and you'll be fine. Love ya, hun. ~MusicLvr98