V.P.
I had my period in 4th grade and was in a bra (and needed it) in 3rd (over 30 years ago!). That's how all the women in my family were. It was not weight - I was tiny and thin compared to my classmates - it was genetics. My daughter, 10, is behind where I was at her age and is also behind most of her friends! If the doctor is okay with her weight and development, I would simply emphasize healthy habits and remind her that as long as she follows healthy habits, she'll develop as she was meant to.