Back when I was 8, I wanted to be just like my dad (before finding out his actual personality). I wanted to have the bodily characteristics of an AMAB person if that makes sense, IDK how to word it. I wanted to be mistaken for a boy, do the stereotypical boy things, reject makeup and dresses, and I went through a “girls stink” phase like some young boys did. I wanted my hair cut short because it made me look like a boy and even wanted to wear my dad’s clothes simply because they were “men’s clothes”.
“Girl" didn’t feel right to me but growing up in a certain kind of family, all I knew was the word “tomboy”, so I used that. But my family tried to convince me I was the most feminine girly girl, and that just wasn’t me. It felt wrong.
As a cis guy, it feels pretty unnatural to say “when I was a boy” because the word has always meant gender more than age to me. Since the variable I’m indicating is my age, I use “kid” or “child” for it.
As to the broader question, my gut tells me to go with your gut. Whether the word “boy” or “girl” is what came to your mind, there is probably some relevant nuance to what it is you’re saying about your younger self that informed that instinct.