Memory loss is something we can all experience. You are correct; it can be caused by stress when our attention is not well focused. Memory loss can also be contributed to by factors such as poor sleep, mood disorders, medications (both prescription and over-the-counter), imbalance in hormones, nutrients or vitamins. Other serious medical conditions need to be considered, so a medical assessment is appropriate for any person with concerns.
In the case of a dementia, where one loses cognitive skills and their ability to function independently, the affected person is not always aware of the changes. Observations and concerns of family members should be shared such that the person with memory difficulties can be gently steered to an evaluation. It is useful to discuss the above mentioned treatable problems as a good reason to see a physician specializing in this area.
We should all strive to maintain brain health, understanding that the choices we make today influence our future.