The Washington State Office of Financial Management (OFM) collects and distributes a variety of data for the state. In addition to the information links below, the OFM site include participation rates, graduation and dropout rates, and other data and reports.
2006 Population Estimates by Age, Gender, and Race
County population projections, by age and sex
Scroll down to "2002 County Projections by 5-Year Intervals", or "2002 County Projections by single year after 2010" and download the Excel file for high, intermediate, or low projections.
County Population Projections by age and gender, 2000-2030 for SVC Counties
Median Household Income Estimates for 1996 - 2007 for all WA Counties