In 2008 Cornell's Program on Applied Demographics redesigned its projection methods and produced population projections by county and by age and sex. The main differences from the earlier methodology and numbers are a 2005 base population instead of 2000, and the way populations in Group Quarters are handled.
Since 2008 the numbers were revised to accommodate new insights, such as new Census Bureau estimates. The results found on this web page were produced on April 2, 2009.
The projections are in 5 year intervals, in 5 year age groups, and project to 2035. The projections are based upon rates of change estimated from historic data. This means that the projections reflect what would happen if the rates of population growth and decline stay as they were. The projections are not meant to be forecasts; forecasts are predictions of future conditions while these projections are meant to gain insight into what might happen if the future looks like the past.
A methodology description and an Excel sheet with the assumptions can be downloaded from the download section.