Paula Spencer Scott, senior editor, writes extensively about health and caregiving. A 2011 Met Life Foundation Journalists in Aging fellow, she helped care for both...
The answer depends on many different factors, including what's causing the dementia. For Alzheimer's, for example, factors include the age at diagnosis, severity of symptoms, and general health status. Nor is there a texbook progression to the disease; some people go downhill quickly after diagnosis, while others can live with the disease for 10 or more years. You can read more about life expectancy here.