“it is acceptable for adolescent children to make their own decisions about their gender identity.”
What does that question even mean?
If a girl saying "I'm a boy" is equivalent to "I'm a Goth", then, yes, it is entirely acceptable for her to make her own decision about that aspect of her identity. It is no different to any teenage experiment or rebellion.
OTOH if 'their own decisions about their gender identity' includes self-harm through binding, compelling other people's speech and humiliating others by removing their access to single-sex services, then, no, those decisions are entirely unacceptable, and adolescents cannot be allowed to make them.