I've known I was gifted since I was 11. We moved to a new school district, and I was tested, identified, skipped 7th grade and was placed in a school for the gifted.
Before that, though, I really had no idea. In fact, I thought I was stupid because I was quite socially inept and couldn't figure out how to relate to other kids. They teased me and called me stupid, so I believed them.
The new school changed my life, and even though I only went there for three years (we moved again,) I had the sense that there were other people like me in the world, and that I did belong somewhere.
Reading other people's posts here makes me glad that I do know my IQ and that it's commensurate with my other test scores and academic achievements. So I don't have any doubts about myself as a gifted person. Still, figuring out how to be myself and live my life is a work in progress!