My Dad is 87 and since Mum died (just under 3 years ago now) his memory issues became much more apparent - often, the spouse is the one doing all the important remembering for the two of them.
He first presented with repetition - and because I'm the only one of the 3 of us (I have 2 brothers who live very near to Dad - I'm almost 2 hours away) who stays overnight, I picked up on the repeating of stories and comments much more quickly.
She also developed something the doctor referred to as "visual agnosia", which may have had something to do with it. Apparently, although you can see fine, your brain can't make sense of what you see. She used to ring me up to ask me the time, claiming the clock was broken and she couldn't find her watch. I got a new clock for the kitchen, and it became apparent that she couldn't tell the time any more.
This was the kind of thing that really alerted my brothers to Dad's obvious dementia - he began to forget how things worked and how to do basic 'operations' as well as not recognising things. For example, I was with him when he said "Where's the phone?" It was right next to him, where it always is.
It was my niece's 15th birthday on Monday and on Saturday I had to do a long chat with Dad about getting her a card, writing a cheque for her present (he can't do buying of presents). By the time I visited on Tuesday, he had absolutely no memory of any of it.