When should dementia patients stop driving?

This is hard. The american acadmeny of neurology has developed guidelines on this which are available online. Anyone with dementia is at higher risk of being in an accident then those without dementia. If someone can pass a road driving test, they are more likely to be safe driving. Most people and those who care about them underestimate the danger.
Immediately. We cannot predict when a pt. With dementia may get disoriented, confused or forget his own address., you may raise the case in favor of pts. With early dementia that preserve their orientation skills and enough memory to get back to the house and to interpret all the signs on the street or obey the traffic rules. You can recommend a real driving test to them to assess his/her skills.