As far as I know, the part of the brain that develops in your 20's is the frontal cortex, which is responsible for 'executive mental processes' (i.e. long-term planning).

I checked out the NPR article, and I think that the parents in that case, and in any case, should treat lightly, and possibly even reconsider their decision to switch pronouns and names. That is a huge step that can reinforce an identity that might be based on superficial, childish feelings that will not carry on over time.

Kids can say and do things for reasons that adults tend to overlook. A child who seems very happy that their parents are involving them in non-traditional gender roles & activities might simply be reacting positively to the extra attention. In that same vein, a child who seems upset about their gender might simply be reacting to newly discovered gender stereotypes.

If they were mocked or told off because they were participating in non-traditional behavior, chances are they were given an explanation along the lines of "because you're a 'insert sex here''.

But kids don't really understand what that means, or how vast its implications are. They probably think that it's merely an arbitrary rule that is limiting what they are allowed to do. Gender roles are understandably frustrating, but they alone are not a reason to transition.

For example, a young boy who likes to play with dolls and wear skirts might end up transitioning, or they might just end up as a man who likes to put on makeup and wear dresses. It's really hard (impossible, I think) to distinguish between these two possibilities at a young age.

This is why recognizing gender dysphoria is so important. It's usually a much more outwardly obvious symptom that adults can diagnose, and I think it should be the deciding factor in determining whether or not a child should transition.