I think I was 22, although when realizing it I know this goes back to when I was 12. I realized it when I was at work. Back then I worked as a system administrator and had to rearrange some cables under a desk, of of them a pretty secretary. While I was rummaging beneath her desk with cables I saw her wiggling her feet, wearing flat shoes, the heel out of it, dangling it on her toes. When I was done I had to go to the toilet. Since then all puzzle pieces fell into place. But only since about half a year ago I've openly about my interest, and I'm turning 40 soon.