Gender identity typically starts to develop at the age of 3. That is to say, three year olds know whether they are a boy or a girl (or something else) and start to act in ways expected of their gender.
Trans children know, sometimes that young, that they are not the gender they are being told they are.
One study shows that receiving gender affirming care reduces the risk of suicidality in trans teenagers between 13 and 20 by seventy-three percent. Another study (I've linked both at the bottom) shows that access to gender affirming care during adolescence reduces suicidal ideation in trans adults by 62 percent.
Denying kids gender affirming care can kill them. They are old enough to know what they are. Trans kids aren't making a decision.
They are not choosing to be trans. They are trans.
https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2789423