Between 10 and 15... (actually when I started "individual" sexual activity!)
I was captivated by two ladies, both older than me: one living in the flat opposite mine, and I could not take my eyes off her when she cleaned her windows, holding the frame with one arm and scrubing the glass with the other hand; the other one with big calves I used to follow on my way to school, especially on the stairs when leaving the underground; I said hello to her once, but she did not reply.
Later on, I met my wife, a swimmer with strong muscles...