in reply to Sarah Brown

That's pop psychology at its worst.

Yes, there is still some development in "the brain" up to the age of 25, but it's mostly to do with the areas of the brain that deal with impulse decision making. The areas of the brain that deal with considered decision making are more or less fully developed at 15.

And despite how things look in transphobic fantasy-land, nobody is transitioning on a whim.