Data; welcome to 2023, where essentially all paths for treatment with diagnosis are available online. Where precise statistics are available. The youngest person to ever have " life-altering changes" was 16.
I also know because I am transgender, I have a doctor, and I am aware of the different steps it takes to get diagnosed and get treated in the first place. I also know that clinicians are extremely hesitant, and it can even be hard to find one to diagnose you.
The whole other side of this argument is ridiculous, and akin to moral panic that has (historically) been used to discriminate against different social groups?

How do I know? I have brothers and sisters, too. My sister doesn't know any transgender people besides me, she goes to school.
It's just not happening.
It's completely made up to discriminate against my people, so forgive me for taking a rather aggressive tone against this argument, but you'd have to be completely bonkers to believe any of this crap.