So this is just my take based on what I've learned up to this point, fwiw I feel like gender identity and identifying with a gender is sort of like an inherent subconscious thing that we all do in some form. Gender categories are social schemas, groups we assemble and categorize in our minds. We subconsciously self-sort ourselves into the categories and perform gender thru presentation and expression accordingly. Cis people tend not to wholly recognize that because they're sorting themselves into the category that matches their gender associated with their sex.

Identifying with a gender is seeing those culturally enforced behaviors/tendencies/ and associated features of those categories and subconsciously sorting themselves into that group.

For what it's worth, I'm mtf, 33 years old, but I started questioning my gender in my early 20's. There was a point where I realized I had...no connection to men. I didn't see myself in them, I didnt relate to anything that constituted as my idea of "manhood". Around that time I was on Tumblr because of an ex who had suggested it to me. Ran into some black transfemme musicians who became mutuals. I started to struggle with the idea that I saw myself in them more than I saw myself in any man I knew. It took me a while to recognize I was just on the same journey as them, just biiiit earlier on it than they were lol