Can confirm, regularly ate 4,000-5,000 calories/day from ages ~13-22. Did some weight lifting for part of that time, a little bike riding, but wasn't a serious athlete, certainly. Was slim, at my slimmest drew comments sometimes because my face was so gaunt people thought I might be sick.

... Then all changed very abruptly (there was little transition, on the order of weeks). Basically had to get used to being hungry most of the time, to keep from shooting toward obese territory. Sucks. I mean, the stuff I was eating as a teenager was going to kill me early in life anyway so it's good I don't do that anymore (so much pizza, snack food, fried food, and soda) but I'm not sure I appreciated, at the time, how special it was to be able to down a large pizza, a couple orders of fries, a couple liters of soda, a bag of chips [edit: and not a single-serve bag...], a snickers, plus a pile of toast or eggo waffles, day after day after day, and still look quite good at the swimming pool.

Some people's bodies at various times in their lives seem to either refuse to incorporate consumed calories or waste them by some mechanism that isn't well understood. Everyone has known someone who is slim and eats a lot and isn't especially active.

But we apparently pretend this isn't true because it's unscientific, 3500 calories in a pound, and all that.

Yeah, I was consuming so much and was only moderately active, that I can't believe it was all going to growth. Much of that time I wasn't even getting taller, or gaining weight, just maintaining. All I can figure is my body was throwing away a lot of it, one way or another. Maybe I had very hungry gut bacteria, which I killed by accident, somehow. I dunno.

Not really. They just burn fewer calories by sitting around when they get older. Calorie count remains constant, burn goes down, weight goes up.

Not really. Physics, biology, and nutrition science all agree pretty conclusively that eating fewer calories than you expend will lead to weight loss.

There are a large number of factors that go into some people seeming to be able to eat large numbers of calories while others can't despite similar expenditures that we're still figuring out, but if you want a 100% guaranteed way to lose weight it is this: track your calories and make sure you eat fewer than you expend. If you want to keep it off, don't eat more than you expend.

Source: lost >150lbs and kept it off until this pandemic fiasco where I put about 30 of it back on.

As noted by others, this is simple. It is not necessarily easy. We're still working on figuring out why some people find it easy and others don't. Gut microbiome science seems promising.

There’s nothing missing. People tend to overestimate their calorie expenditure and underestimate their calorie intake.

Also, metabolic rate slows down with age.

But the equation is the same: calories in - calories out = calories kept.

The equation still stands. If they are gaining weight in their 40s onwards they are burning fewer calories than they are consuming for whatever reason. Metabolic changes perhaps.

Nothing is missing. They're just burning fewer calories as they age. I'm not saying that metabolism isn't a factor (and that changes with age), it's just not something that eating more x or less y is going to change in any meaningful way.

It can, through the second order effects you mentioned yourself. Eating more x or less y can definitely help you reduce your calorie intake without feeling hungry all the time.

What if x is amphetamines?

What’s missing? People in their 40s and 50s have a lower basal metabolic rate.

It seems like you're assuming that rate of burn is constant with respect to age. Perhaps I'm misunderstanding, though.

Your metabolism can change during your life. Different people can also have different metabolisms.

Exercise != burning calories. The efficiency of your metabolism isn't constant, and changes in response to both age and fitness.

Calories in vs calories out is nothing more than the second law of thermodynamics.

Calories aren't equal, otherwise you'd feel sated for the day with 2000 calories of sugar (spoilers: you don't). Every year, we're finding out how much we still don't know about the body regulates our fat stores via hormones. Dr. Jason Fung is a leading researcher on this (https://www.youtube.com/watch?v=ZKC3hiyLeRc)

A calorie is a unit of measure.

Satiation is how you feel... It might change your ability to regulate how much you consume, but it doesn't change how much you gain or lose weight, because all calories are equal. 900 satiating calories add up just the same as 900 empty calories.

The second law of thermodynamics is defined on closed systems, so unless you are the type of person who doesn't eat or breathe then it isn't really applicable.

It is rather like saying E=MC^2 is good diet advice.

That's not to say CICO isn't true, but trying to link it to base physics is a red herring.

I mean, not according to the laws of physics.