Kids who transition (and take puberty blockers) as soon as they're aware of who they really are have a much better time of it [bigot encounters not counted] with better mental health, better performance in school, and generally less incidents of depression and self-hatred.

You can become aware of who you really are at any time, and it generally starts with being completely miserable about your assigned identity. Some trans kids have "always known", some take a while for the metaphorical light to dawn. Either way, their self-identity is important and valid.

You can choose your religious beliefs. You can choose your friends. You can choose where and how you live. What you don't get a say in is your identity and your sexuality. Does anyone remember choosing those? No? Then you shouldn't worry about what or who people say they are.