A lot of people consumed more calories in their 20s and 30s than later in life. Yet in their 40s and 50s they gain weight, with the same sedentary lifestyle or even after taking up a sport.
Something is missing in your equation ...
... 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.
But we apparently pretend this isn't true because it's unscientific, 3500 calories in a pound, and all that.
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.
Also, metabolic rate slows down with age.
But the equation is the same: calories in - calories out = calories kept.
Calories in vs calories out is nothing more than the second law of thermodynamics.
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.
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.