Agree that it's fascinating to think about when I knew my D/s orientation. Sexually, or erotically, my fantasies most of my life have been about anal submission or enemas. Many of my thoughts were about a LTR where we were mostly equals except for sex. Probably this identity started young. On the other hand, I have always been the class academic lead, the lead at work, in charge, Dominant and a decision maker.
I didn't really take hold of, or understand, my identity as a submissive until experimenting with spanking in my 40's; although I've often thought about being 'taken.' In fact, I'd try to brush off my BF in bed, pushing his hand to the side after teasing, thinking "just take me, please." He didn't catch on.
My current (and forever) Dom started out as my Dom.; designed to be physically D/s - and it was for a very long time. Emotionally I still and always feel I'm his submissive, which makes me feel like a woman like nothing else can. However, we started operating as a switch/switch couple, exchanging enemas and other naughty ideas. In wanting to please him, I also found out that Topping him made me VERY wet! And sometimes I even enjoy my sassy Dominant comments to him, spanking him, restraining him. I must say that surprised me, but this happened well into my 50's.
My insight is that I have parts of me that are the same, have been the same, since I was young; other sides of my personality have been malleable. For me it is also partner dependent.