What are the dangers of leaving a 6 year old alone with a loving grandparent who has some memory loss?

Who's sitting who ? Some cultures consider kids competent to self monitor at 8y, but here most consider the teen yrs the earliest. An impaired adult may or may not fit expected supervision requirements depending on degree of impairment. If they are well functioning enough to live alone, recognize danger, obtain help are all needed skills. The child should not be relied on to perform those functions at 6.