You know, I have no idea when it actually started. For a long time as a kid I was somewhat 'overweight' - not the largest kid you've seen, but pretty chubby nonetheless. I remember when I was about 9 being forced to do a stupid dance in compulsory performing arts which required us to be shirtless as part of the costume (nothing seedy - it was for an 'indigenous' type dance). Another boy looked at my nipples and said 'they are really big'. I remember being self-conscious of them even further back than this - as if I always knew I was somehow 'different' and couldn't just take off my shirt like other guys, so this comment stuck with me even more, as did many others which came afterwards through late primary school / early secondary school.
I realised I might have gyno when I was about 15. I lost A LOT of weight and became extremely slim. I worked out partly in an effort to get rid of what I thought was merely 'man boobs' due to my weight and being unfit. Needless to say, the fat went away, the 'boobs' did not. I started doing some research on the web and realised I might have gyno.
I really have no idea when the actual 'onset' was though - for the longest time I honestly just thought it was because I was chubby. I wish that was just the case - it turned out a whole lot more complicated than that.