This is actually something I have been trying to figure out. I can't remember vast chunks of my childhood mostly because that's how my memory works, and partly because I blocked some of it out.
I lived on the end of a relatively secluded road, where the only other kids were either way older than me or abusive/thuggish. I was pulled out of public school halfway through 4th grade after a series of sociopathic teachers made it clear their priority was on playing power games with children rather than on education. I remember having all of one friend - a boy in my neighborhood. And I messed that up by having absolutely zero social skills.
So, from age 10ish I was homeschooled and alone. My social role models after that were my parents, and TV. Before that age I have very very few memories related to gender, all inconclusive... I remember being very angry when my parents made me cut my long hair for school photos, I remember sitting on my junk to make it go away, I remember riding in a car and thinking I smelled "like a girl" and being happy about it. That's about it. I did not attach any significance to those early memories, I didn't think anything was wrong - I was just a kid, and I was happy.
After I got pulled out of school and puberty simultaneously started raising its ugly head in earnest... I had no real frame of reference for what was wrong other than TV and the internet. So my dysphoria expressed itself as ANGER and FRUSTRATION... which turned into a pointless grudge I harbored against my parents, which prevented me from telling them. A fact which I will always regret because they would've helped me.
My parents wanted me to go to public high school for the experience... but I absolutely refused to allow them to register me as a boy - and I wouldn't give them that as a reason, so I just flew into a rage. So I got a correspondence school for high school. I remember being so sad when they signed me up for that, because it was still as a boy...
So, I would have to say somewhere between 10 and 12 I figured out something was wrong, and between 12 and 14 I figured out WHAT it was.
Sorry for the vent session, it takes a lot of effort to remember this stuff in order, helps to write it out.

Now I want to go hug my mom, because I know she blames herself now for not "being there for me", and it wasn't her fault... she can't read minds.