They are not talking about cognitive decline, they are talking about speed of processing. I am mid 60s, engineering for the last 38 years, and the depth of work is not a problem, but I definitely don't make the same snap judgements that I made even just ten years ago. It also pertains to physical reaction time. Meaning, after 60 you best leave a little more gap between you and the car ahead.
You may doubt this now, but you will see how life changes for you after age 50, and again how life changes for you after age 60.
It is all fine, it really is, but you will find yourself saying "that didn't used to hurt", and you will also find yourself shoving people away from your mouse because they want to click on something that you haven't decided to click on yet.
It's just nature. Yes, some people abuse their bodies and their minds, but even the most careful are going to see changes. Just life, just how it goes.