Oh, good discussion guys. I think this is a really interesting question that for me dosn't have a clear answer. Like they say, hindsight is 20/20...so when I look back to my very earliest sexual thoughts and fantasies I now realize that they all revolved around my being controlled somehow, weither it was being tied up, kidnapped, etc. it was always there.
It's difficult for me to pin down when I "knew" what the implications of those fantasies were though, there was never a point when I "discovered" I was a submissive. Because I've always had those feelings as far back as I can remember, I think my understanding of what they meant evolved very gradually as I was growing up.
The first time I heard of BDSM and dominants and submissives as labels was when I was 9 or 10 and my mother gave me THE big talk, explaining sex, in all its many forms.
Of course my understanding of actual BDSM practices and lifestyles changed over the years as I got older and curious-er and searched out more details, but I think of that as a very organic process.