This is a great question and one that is very applicable and practical. The first point to keep in mind that there is no one answer, because all people are different. Hence, you would need to treat each person individually. With that said, there are several possible reasons for memory loss in old age.
First, as people get older, sometimes they get depressed. When this happens, people can lose some of their memory. There is a correlation. Second, as people get older, the more they are on medication. Some medications have a side effect, which reduces memory retention. So, you should consult a doctor about the side effects of medication. Third, thyroid problems can make a person anxious and this too can cause loss of memory or difficulty in recalling things. Fourth, more seriously as our minds and bodies get older, there can be dementia. This make a person very forgetful of things. There are, of course, many other factors, but these are some of the most common.