This is actually a very active area of research.
There are a number of factors going on here. Kids are less active. Lower activity means fewer calories burned, and higher probability of gaining weight.
Once the weight starts to go up, the metabolism changes. It then becomes very difficult to lose weight. Every time one tries to cut back, the body starts to send all kinds of signals that it’s starving.
I used to ride a bike to my friends house and play ball till it got dark. Than my mom would call me in.
Now I just work all day for the privelege of the gubmint to tell me what to eat, drink or smoke.
The nanny state is a nightmare that would even scare Freddy Krueger.
Yep, I drink diet coke and am considered mildly obese although I know many much larger than I. I was rail-thin until I hit about 23-24 years old, then put on about 100-120 pounds in 15 years, much of that relatively quickly after turning 25. No change to diet or activity level - just a metabolism slowdown. It was like a switch in my body was flipped. Don’t get me wrong, I love to eat. I drink diet coke because I love it. I can’t drink the sugary sodas at all. SO as much of that as I drink is 140-160 calories that I am avoiding per can by drinking DC. I can only imagine how much more I’d weigh if I drank regular Coke. My oldest daughter is slightly overweight and doesn’t eat much at all - not like I did as a kid. My wife is not fat, so I think genetics may play a role, or there is something else we just don’t know about.