E.g. when I read a novel, the last few pages don't fly by any faster than the first ones do.
At any rate, I’m old and I can confirm that the speeding up isn’t just a myth.
The theory in the article did strike me as quite plausible.
"Cognitive decline" is probably the wrong way to think about it. Your cognition changes as you age.
When you're young, you're inundated with lots of information. You can process that information quickly, but you don't know which signals are important.
As you age, you learn what information is important, and what can be filtered out. So even if you're slower, your brain can work more efficiently. It also knows how to take shortcuts.
...at least, that's what I remember reading in my college Psychology textbook five years ago, and it jives with the current article.
For that 50 year old it was just 0.03%.
You gain weight because you eat more calories that you burn. This is, again, mathematically correct, but it offers no explanation of the why
Why do you eat more than you burn ? you move too little ? you eat too much ? your brain malfunctions and it is unable to detect food intake ?
(Maybe I misunderstand your angle).