I'm a high school P.E. teacher and coach 3 sports so I work with literally hundreds of teenage boys. Let me tell you its way more common than you realize. Many kids regardless of size or body type exhibit the gyno-look. I'm not sure its even gyno we're looking at. Some peoples' nipples are just made different. I think that way to much attention is being given to this topic.
In todays world, I think it's much more prevalent. We see it more now due to the obesity factor, and depending on how you feel about the current food industry, some blame the hormones put in animals for causing this and other assorted maladies found in kids today.
Nintendo was popular when I was a kid, but todays kids seem to plant themselves on the couch and do nothing but eat junk all day...lots of it.
At least when I eat/ate junk, I would go out and burn it off playing outside, or with friends, etc...