And if their gender identity is insistent, persistent and consistent by the onset of puberty, we can be almost completely sure that it is fixed. At that point, we move, in some cases, to puberty blockers
Physical puberty in girls can start between the ages of 9-12 - many of the teenage girls who are questioning their gender identity are actually in the 13-16 year age group and have gone through their physical puberty as puberty sparks a huge amount of neural development on which many, many child and young people are forming their identities, uncertain and often feeling anxious and depressed.
There's really very little understanding of puberty or adolescent brain development, which is very worrying.
There are many, many stories of people who found their identity to be in flux in adolescence and it only became 'fixed' as they came out of their late teens.