Here's a paper on "gender-affirming" mastectomies from researchers in the US. The cohort studied includes girls as young as 12 years old at the time of surgery: https://journals.lww.com/annalsplasticsurgery/Abstract/2022/...
Also here's a piece from Reuters about rates of "gender-affirming treatment" in children, taken from health insurance data - the section for "top surgery" includes girls as young as 13: https://www.reuters.com/investigates/special-report/usa-tran...