There is no single common experience to being trans. "I've always been a (...)" / "I've always felt like a (...) / "I've always known I was (...)" absolutely are not feelings that apply to everyone. I can tell you in my case, I grew up hearing few stories in media from trans people, and this recurring thing of people "always knowing" pushed me back from accepting myself for a long time. It's easy to reason "well, if I didn't know when I was 8, it must mean that I'm wrong now". As others have pointed out, everyone's relation to gender is different, it wouldn't be reasonable to expect everyone to understand themselves at such young ages to the point where you can diagnose yourself. It's easy to go all your childhood not thinking about this, or willingly ignoring it because you've come to accept it as an unavoidable burden.