My daughter is worried about her weight, I told her not to be, but she still is. She's 15 years old, 5'2, and weighs 121 pounds. Is that average? I'm

The range of normal. It is certainly common for teens to be sensitive about anything "self" & weight is a big cause of stress for some. Looking at others her age sets the standard, but that is confusing. The wide range of normal at her age gives rise to mis-identification.The normal range of weights at her age/height range from 93lb on the thin end to 132lb. Yes she is normal, but that may not stop her worry.
Body mass index. Her body mass index (BMI) is 22.1 which puts her in the normal range for weight for height. Reassure her that her weight is average for her age.