@Archery Annie. You are right, nobody was talking about telling a child they were being silly, I was picking up on the comment so silly that he has to identify as transgender in order to wear sparkly clothes. It would have been better to have focussed on thinking of it as being 'silly'.
@StarkStaring. It is complex - in psychological terms many psychologists, me included, would see under 25 year olds as adolescents still. The developmental task of coming to a settled sense of self is often still on-going at this age and so, in my view, struggles with gender identity could still be part of this process. I am not convinced that someone who is struggling will not come to
a settled, consistent identity that is at one with their sexed body, given time.
Having said that, I think the age of onset it important. I think there are significant differences with persistent dysphoria that starts at a young age and sudden onset dysphoria (where social contagion aspects could be at play).
What some posters on this thread may be overlooking is that some children, from a very young age, present with passionately uncompromising narratives about their self-identity as male or female, despite the conflicting evidence of their sexed bodies. When talking to them, it is clear that their understanding of their own sexed bodies and of gender is complexly determined.