I'm just gonna go and say it.
the age from which children should be allowed to change their name, their legal gender, and pick their own pronouns is 0.
they're not going to develop that wish before they have developed a sense for how gender works, and from the moment that they have, they should be able to be who they want to be.
sure, allowing even small children this type of freedom would mean a lot more experimentation, going back and forth, reversing choices, and would require a much more fluid understanding of names, gender and identity.
but why should that be a bad thing?