Some people say they have known since they were 5, for me though I started suspecting, but not really taking seriously, that I am really a girl on the inside, around age 14 or 15. I just started taking these feelings seriously this year. Sure there's small hints of doubt that come up in my mind, but most of that is because I don't feel like a stereotypical girl, but as people have told me here I don't have to be like the majority of girls or do a lot of what is considered "girly" in society I guess. Anyway how I know is that over the past few years, every or at least the majority of days, I have fantasized a ton about being a girl. Mostly as society seeing me as one and of course the physical aspect of it too, as in imagining me having a girls body made me feel happy. At first I thought it was all just attraction, which yes I am attracted to girls, but also desired to be one and I only realized that deep down I wanted to be a girl this year, even if not really girly or stereotypical. Once I realized I am a girl on the inside I have felt a lot of physical and social dysphoria which I am trying to live with. Before that I was ok with my body, but now I am definitely not. Maybe accepting you feel like the opposite gender instead of trying to fight it kind of amplifies dysphoria some? Anyway I hope this helped.