I had on on my left side only from the age of about 13 or 14 onwards (I didn't start training until I was 16).
As I got bigger, heavier, and added bodyweight over the years, the gyno got bigger too and when I started taking gear in '02 it got worse still. My right side was unaffected throughout this whole time - no idea why.
In January of 06 I had it surgically removed and had 100 grams of hardened tissue taken out. I also had liposuction on the right hand side too just to balance things up a a bit. I've been back to the gym since then (it took two months to heal properly) and I've had no problems.