The Washington Secretary of State's website provides a lot of interesting information about who voted in the most recent election. Go to
https://sos.wa.gov/elections and click on the Data & Research tab. Under Election Data & Maps, check out "Current Election Ballot Return Statistics". For individual County statistics, select the tab of interest, and then select the county from the left hand scroll bar. For example, statewide 62.2% of voters 65+ cast a ballot; in Whatcom County, it is 67%.