That's a good devil's advocate, but it's worth pointing out minor children consent (with parental consent, too) to surgeries all the time. Whether that's cancer treatments, surgeries to repair bone fractures, plastic surgeries, etc.

I believe bottom surgeries are all but unheard of for under 18, but there were 203 top surgeries performed on minors in 2021 (safe to assume vast majority of these were either 16 or 17). In comparison, more than 8,000 minors had breast augmentation in 2019. 4,700 girls had breast reduction surgery in 2010 (apologies, these were the newest numbers I could find, clearly after more than a decade it's reasonable to believe there are more reductions than implants). In 2015, there were 7,021 breast reductions for boys.

It seems to me like if the genuine concern was over minors making decisions about their body that aren't reversible, we would be seeing equal and proportional outrage for these other surgeries, right?

Edit for your edit :) Gender affirming care appears to be what's best for kids according to the majority of research and the major medical associations in the US. The experts certainly could be wrong, but generally speaking I prefer having doctors treating me in a hospital and pilots flying my planes.