Jules, some cognitive difficulty may be aging, but my cognition is not worse than it was 30 years ago when I was at the height of my neuro (undiagnosed) difficulty. I completely lost the ability to think at more than a pre-school level when they put a toxic rug in my college where I taught. So I had to quit that day. I couldn't even wait. I had to let them get someone to take the class. I had had difficulty with lights and other things before, but this was the last straw.
At that time I was tested cognitively, and my IQ had gone down about 18 points. I think it went back up. I never stopped attending Mensa lunches, at least when I could get there in the car, when I improved enough to drive again, after the horrible rug incident.

I have what MS pundits call "loss of executive function". It means I can't "file" things. This makes doing my records and taxes a misery, but I do them all in increments and don't scold myself when I have trouble. That's the important thing, not to blame yourself when you lose cognition.

Today was a bad cognition day. I had to deal with H and R block over a tax glitch from 2010, and I left my wallet at home because I was unable to coordinate everything very well. I made several cognitive boo-boos, but I continued on and just told them I had a neuro illness and it slowed me down. And they believed me, I think, because sometimes I'm "sharp" and they can see that this comes and goes.
But I felt sad coming home alone, still whacked in the brain but hoping.