Just that simple. The memory isn't what it once was. This in itself causes untold problems. Without recall, discerning reality from fiction is much more work as there’s little to grasp onto in order to make good decisions.
Since their brain can no longer be trusted, it’s up to others to take matters into their own hands and do what they need to do. It’s no different than if they were on drugs or drunk. Impairment is impairment.