I would say my freshman year in high school is when I first started getting noticed (age 14), though I really have no idea why. I was so awkward and shy of girls then. I'm only saying 14 because that is when I started dating, and both girls I dated that year were very attractive and popular (I only mention popular because it actually mattered in high school). Sophomore year, I didn't even have to try to land this smoking girl (still no idea why she dug me, because I was still a total dork).
I think junior year (16) was the year I really started being confident with girls. I had actually moved to a new school, and got a lot of attention (probably because I was "the new kid"). This is when I started gaining the most size I think, and actually had a decent build. I think it was a combination of the looks and personality that got me attention. I also got in a good clic of friends.