There are some diagnoses that can only occur once someone reaches (legal) adulthood.
Given that the majority of children with gender dysphoria desist by age 18, what would be lost by making this a post-18 diagnosis?
I could also see a possibility for there being a paediatric diagnosis, possibly with different criteria, but certainly with different recommendations (watch and wait rather than affirmation, for one).
I'm wondering what there is to gain by diagnosing children with something they normally grow out, developing treatment plans etc, assuming Big Pharma is removed from the equation.