There is no one answer to this question as it can depend on each individual case. In general, dementia is a progressive decline in cognitive function and ability to perform daily activities. As the disease progresses, a person with dementia may become increasingly unable to make decisions about their own care and treatment. This does not necessarily mean that they are considered incompetent.
In order for a person to be declared legally incompetent, they must be assessed by a mental health professional and found to have impaired judgement and decision-making skills. This assessment must be done on an ongoing basis, as cognitive decline can happen gradually over time. If a person with dementia is found to be incompetent, they may lose the legal right to make decisions about their own care and treatment. However, it is still possible for them to participate in making these decisions as much as possible.