At which point is medication for dementia necessary?

For symptoms. Dementia cannot be treated directly. There are medicines for many symptoms or interventions involving the environment or training caregivers. Dementia specific medicines like aricept, namenda, Exelon (rivastigmine) can help prevent or treat anxiety, agitation and aggression.
Dementia. Cholinesterase inhibitors such as aricept, exelon, (rivastigmine) and razadyne in combination with nameda should be started as soon as dementia is suspected, especially if alzheimer's is believed to be the cause. They provide the most benefit if used early. Later in the illness, various psychotropics can be used when the patient begins exhibiting impulsivity, aggression, apathy, or psychosis.
Dementia therapy. There is increasing evidence that the earlier medication is used for dementia, the more success will be achieved. Once the diagnosis is made, a medicine specific or approved for that type of dementia should be started. Although the "memory enhancing agents" are not curative, they can slow or delay the course, and maybe control some of the behavioural symptoms.