The thing is, I have an issue with this concept, i.e. someone knowing they’re trans from a young age (like this is the definition of “real/true trans”, as opposed to the fetishists and incels).
It’s a mental health condition. Any individual with enduring gender dysphoria from a young age needs help. Because if we accept that one cannot be born into the wrong body, then you need to investigate other causes - autism, abuse, trauma, etc. And they need help accepting their body as it is, not encouraged down a road of being a permanent medical patient, living as a facsimile of the sex you want to be.