"My mother cannot keep track of her appointments. She continually mixes them up, and it is rather frustrating to always correct her.  It can be maddening!"
 

This is a complaint that is often heard by home care professionals.

Lucky the child whose parent is as sharp as they remember them in their youth.  For the vast majority of us, that will not be the case.

The aging brain is not as agile as a young one, just like the body overall.  Wisdom often compensates for our less able bodies as we develop skills and more efficient ways of doing things.  While your brain may have fewer cells at 50 than you had at 16, I doubt that any 50 year old would believe for even a moment that their 16 year old is smarter than they are. 

Now let's fast forward to age 85.  You do not hear as well, you may not have the vision you once had, your memory is not quite as good, and you no longer go through the mental gymnastics that your working brain had to deal with daily.  You repeat stories that you have already told.  You forget what you have been told. You ask the same questions a few times.  To the able mind, you are frustrating.  You are also frustrated because you may realize that you are not remembering.

This is one time when I believe that things must, must be reduced to calendars and writing.  Verbal communication will simply not work.  If mom reads email, send all established appointments in an email.  Also place all appointments on a calendar.  Make sure there is a device in the home that identifies today's date and day of the week.  Ask your mother to write all appointments on the calendar upon receipt and practice repeating the date and time back to the person making the appointment.  It is a good rule of thumb for all of us to do this.

Now for your frustration.  Do your very best to be amused, not frustrated.  We all mix things up.  Be grateful that your mother or father is still with you, and forgive the side effects of getting old, as he or she likely forgave you for the side effects of being a teenager.