I'm gonna try to reply to this in good faith as you seem to be wanting to engage in a good faith discussion, so let me tell you my experience.

I'm a trans girl.

Before I even have memories I was always (according to my mother) asking for skirts and dresses and playing with dolls and makeup. These moments were always taken as a joke or as me 'being in a phase' and were brushed off and ignored. My earliest memories have me confused why my sister gets to wear pretty dresses to church but I have to wear a boring suit. I remember 'borrowing' my sister's nail polish, makeup, and dresses as early as age NINE. Did I have the understanding of what I was feeling? No. This was in the 90s, trans people werent as widely known in the US, especially the South. But if you'd given me the option to transition at age 9, I would've taken it in a heartbeat.

How do I know?

I'd slip out of my window EVERY NIGHT after everyone had gone to bed to wish on the first star I laid eyes on to be a girl. Sure, it wasn't the first star to appear, but it was the first one I saw! That counted, right? I remember watching the episode of Sabrina the Teenaged Witch where she made a potion to turn herself into a boy to figure out what her boyfriend did in the auto shop. What did I do immediately after the episode ended? Went into the kitchen and tried to make my own potion with 'sugar and spice and everything nice'. Glitter is nice! Oh and soda! Soda's very nice.

I went to bed daydreaming of going to hogwarts and finding a potion that could make me right. I dreamed and dreamed and dreamed for a way to change my gender that when I finally heard about trans people at age 17, my reaction wasn't 'hmmm, this is interesting. I wonder if this is what I'm feeling', instead it was 'Oh, THAT's what this is? There are other people like me?!?!?!'

Scientific studies have shown that a child's concept of gender is already developed by age 4. Studies have also shown that a child's understanding of who they are ALSO develops pretty early on. And, yes, in cases of children going on hormones, it is all done in conjunction with the child's experiences, the doctor's EXPERTISE in the matter, and the parents' consent. They aren't just walking into a doctor's office and boosting little timmy up with E. Before any medicine has been taken there is EXTENSIVE screening through therapy and physician visits over the course of YEARS.

This isn't a question that nobody has ever asked. It's studied. It's tested over the course of DECADES. And lastly, it's (frankly) none of your business unless it's YOUR kid, at which point you have complete control over whether your child goes through with any of it (up until age 18 at least).