Dementia is the between the 3rd and the 5th largest cause of death in first world countries, depending on the country, and how you count. Most people aren't currently experiencing dementia, but there's a very significant chance that you're going to end up with it eventually.

The actual question should not be what percentage of the population has it; the question should be what percentage of people will be suffering from dementia to the point that they need assisted living care when they die.