I am from Australia, I am also very active ( long-distance runner) I run 5-6 marathons a year and I train 7 days a week. I'm 23, been doing it since I was 15. I've always been healthy as in I don't drink alcohol or smoke or do recreational drugs and take-away for me is a once in a blue-moon thing.
To the main point ~
When I was in high school, a public school of about 1200 people. There was barely any overweight or obese people. In my "house" which was about 1/4th of the school, there was probably about 20 fat kids, out of a couple hundred, and maybe 2-3 were obese. But I am hearing figures that 50%+ of people are overweight/obese in Australia.
What I did notice is, the majority of kids at school, who were thin, didn't actually play sports or do any sorts of activity, the majority being super-lazy, not walking/riding bike to school, most of them got picked up. And to top it off most people ate junk food, drank a lot, and smoked. I was one of the odd ones out because I was always eating healthy food (never from the schools snack shop) and was just generally far more active than everyone and stayed away from alcohol.
So based off the "50% of Australians are overweight or obese". I assume these are counting mostly people in their mid to late 20's, 30s, 40s, 50s etc. While the majority of "teens" are still thin and at a healthy weight in their teen years, I assume their lifestyles of being inactive and eating junk catches upto them when they hit their late 20's/30's.
I have a cousin who is 19, he is thin as a stick, he doesn't do any exercise at all, drives everywhere, he eats take-away (mainly mc-donalds) several times a week. He brags about how he can do that and he never puts on weight let alone gets fat. Is he heading in the direction of overweight/obese when he is in his late 20's or early 30's?
I am bringing this up here, because I see a lot of people who post in these forums in "fitness/heath" sort of topics, about how they're thin and eat whatever they want and never get fat. It's interesting to me, because if you notice at gaming events...there is barely any middle-ground in weight differences. The people at these events are either A) Really thin, or B) Really overweight - Both types of people pretty much live the same lifestyle - junk food/inactive, but one is thin and one is fat, not to mention they're all around 18-25 years old.