My curiosity is if this is like you suggest, ingrained patterns, or if there is actual slow down with age. I hear different opinions and am finding it difficult to navigate as I deal with my own, albeit mild, aging.
I actually noticed serious mental decline when I was burned out in my late 20s. There were real physical symptoms like not being able to look at a text editor for more than 2 minutes. Post recovery of that, I actually feel like my brain recovered a lot once I started learning languages very seriously (mandarin and japanese), starting a few years ago. Brain feels healthy now but I'm acutely aware of where it's not as sharp as before. Playing around with this felt a little like when my brain is trying to build a new grammar dictionary.