This is especially interesting given a common anti-transgender claim that people should not be allowed to transition "until their brains have finished developing" (i.e., 25 or so). I hesitate to call it an "argument" because I don't think the concern about brain development is fully sincere. I think the anti-transgender people just want to delay other people's transition as long as possible in the hope that some trans people by age 25 will give up on the aspiration to transition and others will be easier to flag as trans because they won't "pass" as well or whatever, or in the hope that if they can control a 24-year-old adult's life choices then they can also control a 34-year-old, and so on. Anyhow, interesting that Rowling's fictional character is an astrophysics PhD at 23, when Rowling spends so much time in real life expressing concern that teenagers don't know what gender they are.