I've just spent a couple of hours with my mum and dad who are both 72. Married for just under 50 years. Both retired for 10. Dad has a hobby, mum doesn't (unless you count cleaning which she does constantly).
My mum doesn't have any cognitive issues but I have noticed she is getting really contrary with my dad. For example - he will say "I remember when we got really cheap tickets to see X musical" she will interrupt with "They weren't that cheap!!" and my dad will say they were £30 which is of course good for a London theatre. So I say that is a good deal, and she backs down. Or he will say they got their kitchen done in around 2012 and she will say "Nope, nope, it was February 2013". Or my dad will say my cousin J posted X on Facebook and she will say "He didn't! He didn't! That was B!" and when I have looked, they both posted the photo - and my dad didn't say it was J only, mum did, so she has caused an issue for no reason. My dad looks a bit exhausted.
I don't see them often due to distance. I'm not sure whether to stick up for him or not. He won't do it himself as there's just them at home and he says life's too short to argue but maybe he should be? Do I just leave them to it - Their marriage and so on? I don't feel quite the same about not getting involved now that they're both getting a bit frailer.