On reflection, I also know somebody who eats like that and still has a high metabolism at 31, so I'll append a "probably" into my statement about Chicken Nugget Girl's weight. Absent extremely fortunate genes, it's also not impossible for her to remain thin without changing her diet if she exercises strict portion control, but I strongly suspect that her age has more to do with everyone's surprise than her willpower.
I was worried I would balloon as I got older and did a little background reading; I tend to blame it on a combination of genes and life history, in particular;
Brown Adipose tissue; fat cells that are specialized to turn calories into heat; almost literally, burning fat.
Infants have a lot of it, adults not so much, but how much you retain depends on how active you are
and how well insulated you tend to keep yourself. From what I understand it's not that easy to get brown fat back once it's shrunk, so if you change your habits and cease some regular exercise or even move into a warmer house, your loss of brown fat means that yep, your basal metabolic rate drops and you burn off fewer calories, active lifestyle or no.
So if you just generally get out and about a lot through your teens and stay that way, you probably keep hold of more brown fat and maintain a higher basal metabolic rate. And can eat like a horse and never show it.
However, move to a different climate, take a sedentary job indoors and all of a sudden, your body gives up on stoking so much brown fat all the time. It's a bit of a vicious cycle, seeing as then you feel the cold more and probably rug up when going out rather than not feeling it and coaxing your body to nurture its brown fat and keep you running like a human space heater.
For me in my late teens it was trivially easy not to gain weight. I did nothing and ate what I wanted. But, fall off the wagon, lose that brown adipose tissue and you have to make the effort to remain active and watch what you eat. Yeah, older may mean a gradual loss of brown fat, but also, more opportunities to spend a year taking it easy and fall off the brown fat wagon as it were.
Anecdotally, I noticed it became a lot harder to lose weight after a one year period in which I both moved to a warmer house and adopted a much more sedentary lifestyle than I had before or since. In addition to being born with the genes for it, I think that all those people who manage to retain their hummingbird-esque metabolisms just maintain their habits that help them retain their brown adipose tissue, while in other people, adopting warmer, more sedentary habits for a year or two drops them into a self reinforcing spiral of feeling the cold and putting on more layers causing the loss of more and more brown fat... Followed by sudden weight gain when their eating habits do not change to match their suddenly lower basal metabolism.
