Do you believe a child can mutilate their body with tattoos, do drugs, have sex, fight a war, put toxins into their body, etc?
Then they can mutilate their genitals and we new to lower the age of legal adulthood.
If you don't believe they should be able to do the above, then they can not mutilate their body.
The only "issue" here is that pre-pubescent children transition "better" (while also regretting it the most) than adults. That being said, people with gender dysphoria are mentally ill, and need professional psychological help. This is why I advise parents to monitor their children's internet access until they consider their children to be adults- tumblr and similar degenerate sites will corrupt a young impressionable mind.
By monitor I obviously mean check their history and talk to them about what's online- not stand over their shoulder kind of shit. Publicly seeable computers only as well. A laptop as a child is what temporarily turned me into an atheist and a porn degenerate autist.