Can dementia be prevented or delayed?

Possibly. The best current evidence suggests exercise may delay or possibly prevent dementia. Eating a good diet may help. If you have diabetes or high blood pressure work with your healthcare team to control it. Research is ongoing.
Maybe/ yes. It depends on the type of dementia. Cerebrovascular risk factors can be changed thus preventing the onset of vascular dementia. The other dementias, which follow a pattern of genetic expression, such as alzheimers, lewy body disease, and picks disease, will invariably manifest themselves eventually. Aricept, exelon, (rivastigmine) and razadyne in combination with Namenda can slow down the process of alzheimer's.