I heard a report that said today's average adult expends about 300 calories less per day than those back in the '60s, I think it was. The cause, of course, is all the automation we have today... although even in the '60s I suspect that, while the "average" adult was still doing more physical labor, there were already plenty of folks with "desk jobs" who weren't.
As for teens today... the human body definitely has some amount of regulation built-in, in that you can readily find teens with comparable body shapes/sizes engaged in comparable activities but where one might consume 20000 calories whereas another might consume 3000 and they both maintain a healthy weight. Of course at some point that system does break down -- I doubt there's anyone (other than, e.g., olympic-level athletes) who can consume, say, 5000 calories a day without rapidly becoming obese, yet these days it's not that hard to come by upwards of that many calories in a day if you're supersizing a lot of fast food meals and keep a bag of "munchies" around.
An alternative point of view might be... even in, say, the '60s, there were always a few teens who'd be overweight on, say, 2500 calories per day, despite performing roughly the same amount of activity as their peers who weren't overweight. ...and unless you *were* one of those overweight teens, you probably never did much calorie counting. Today, the average caloric intake probably is higher -- maybe, say, 300-3500 calories per day --, so you see that many more overweight teens.
It's kinda unfortunate that the stomach respond more to bulk to measure fullness than nutrition, whereas the palette tends to prefer sweat and/or fatty foods that are highly calorically dense but not very bulky. There'd be a lot fewer obese people around today if a Big Mac only contained 10 calories... (yes, some people would just eat more Big Macs, but many people I think really are "eating for volume" more than "eating for calories").
---Joel