Dementia may accelerate your parent's physical and mental decline.  If your parent has been medically diagnosed with dementia, it does not mean they have to stop driving that day... but it does mean you may have to pay more attention to their driving behaviors.  Dementia can be sneaky.  For example, if you mom or dad is heavily affected by Sundowning, their ability to drive at dusk and into the night may be significantly impaired... but if your test drives with them are always on Saturday afternoons, you may never see this set of problems.