Childhood gender-nonconformity isn't the be all and end all of being trans. Yes, most MtFs are to some degree more feminine as children, but not everyone. Cultural pressure to act a certain way, and a childhood lack of understanding of what gender really is in the first place, mean that it's a bit more complicated.
You seem to understand that others who are not trans in any way also express childhood gender nonconformity, and you're right. Among those who were admitted to therapy for childhood gender nonconformity, only 10%-25% eventually identified as trans in adulthood.
Early puberty is the key. That's the age where most people grow out of their childhood gender nonconformity, and accept their identity as their birth gender. But for those who are trans, the opposite happens. They start developing body aversion, and their dysphoria gets stronger. In a study of teenagers who identified as trans strongly enough to be put on puberty-blocking hormones, 70 out of 70 retained their trans identity into adulthood. So currently, according to WPATH, the most accurate marker of whether one is trans or not is how they respond to puberty.
The reason I always say this is because when I started transition, I was really scared that I was making the wrong decision, because I pretty much had no childhood gender-nonconformity at all aside from always having girls as best friends. Had you asked me about being trans at that age, I would have laughed and said "no way." But then as soon as early puberty hit, suddenly out of nowhere I started developing trans tendencies. I started hating the body hair that was coming in, was scared to death of my voice changing, started hating the pictures that were taken of me because I hated looking like a developed male. I started hating my genitals, and started developing more feminine social tendencies, and hating being treated like a guy. So despite believing that I was trans for almost certain because of my teenage experience, I went through a lot of doubt simply because I didn't have that same sense of "I've always been a girl" that others say they had. So it was a great relief to know that WPATH acknowledges that one doesn't need to have any childhood gender-nonconformity whatsoever in order to be trans, that the real signifier was the early-puberty years. And the APA's official diagnostic manual (which I'll get to in a moment) makes no mention of age whatsoever.
Now I kind of look back at my childhood and see it as though I myself was one of those tomboyish gender-nonconforming girls who thought that I was a boy as a kid, then hit puberty and all of a sudden grew out of it, and wanted to be feminine. The only difference is that for me, I couldn't be feminine, because my body betrayed me and insisted on making me more masculine.
So yeah... I appreciate that a lot of people on here knew since they were kids. But I think it's a problem when anyone tries to apply their own experience to the population as a whole. Some knew since their first memory, some didn't know until puberty, some didn't even know until later. According to WPATH, gender-nonconforming kids definitely aren't necessarily trans, a lot of them grow out of it. Teenagers who identify as trans pretty much are trans. And while it doesn't specifically mention anything about those who didn't develop a trans identity until adulthood, I would like to share the following:
According to the DSM diagnostic book, here are the only requirements to be diagnosed as trans:
"A marked incongruence between one's experienced/expressed gender, and assigned gender... of at least 6 months duration, and manifested by 2 or more of the following indicators:"
1. Incongruence between one's experienced/expressed gender, and one's actual primary/secondary sexual characteristics. (We call this "body dysphoria.")
2. A strong desire to be rid of one's primary/secondary sexual characteristics due to this incongruence. (Or in young adolescents a desire to prevent the development of anticipated secondary sexual characteristics.)
3. A strong desire for the primary/secondary sexual characteristics of the other gender.
4. A strong desire to be the other gender. (Or some alternative gender different from one's assigned gender.)
5. A strong desire to be treated as the other gender. (Or some alternative gender different from one's assigned gender.)
6. A strong conviction that one has the typical feelings and reactions of the other gender. (Or some alternative gender different from one's assigned gender.)
That's it. At least 2 of those indicators, persisting for 6 months or more. It makes no mention of at what point in life these things must occur. Just that they need to be there. And according to the APA, these criteria yield a true positive rate of identifying gender dysphoria 94.2% of the time, and a false positive rate of only 0.7%.