I was a tomboy and often preferred to play with boys, loved sports especially football, hung out with the boys when they let me (3 of my first four friends were boys) and didn't particularly care about clothes, played with Tonka trucks, but tended to wear whatever my mom dressed me in. We lived way up in the hills in a rural area, so that usually meant pants and t-shirts. I had really girly swings too when I LOVED my pink polka-dot frilly dress, playing with Darci dolls (a Barbie competitor in the 1970s) and adored horses, and "decorated" and made "clothes" for my horses.
But I never really thought about what it meant. My mom was a super-hippy dippy chick and was trying to raise me without gender expectations (even made sure my name was unisex), and I played soccer with other girls, so they must be the same, right?
Hindsight being 20/20, I realize I was already showing gender swings, but since non-binary type gender identities weren't a known thing (we lived in the San Francisco area, so transgender and homosexuality were no big deal) it wasn't a concept for me to consider.
When my hormones hit at about 11, I did start to realize *something* was wrong/different about me, compared to the other girls, but I couldn't figure out what. It was also about the time I began to fantasize about having male parts. But since I didn't want to give up being female, trans made no sense. I never thought I was anything but cis - just a kind of weird cis - because I had no other known option to consider.
I think puberty is a more common time than childhood for people who are trans of some kind to begin to really feel their gender identity, if it doesn't match their body. As a child, life isn't as gendered, unless you have family/community that really pushes gender conformity. Without a strong frame of reference for gender as a child, it wouldn't be a big deal until hormones hit. And the hormones make it much more clear as your body parts begin to change - either as your brain expects, or in the opposite direction from what seems right.
I really don't understand where "adult-onset" transgender people come from, as far as brain biology and stuff goes (I tend to be science-minded, and consider nature more influential than nurture). I can't figure out if it's all simply a lack of understanding of who/what you are (like myself), long-term denial, or if it's something mental that clicks later in life.