Ben,
In order to use the CORREL function (correlation function in Excel), make sure that you have two columns of data, with each row representing a county:
column 1 --> for each county, the percentage of voters over 65 years of age
column 2 --> for each county, the percentage of democrat votes
Assuming %dem + %repub = 1, it would be redundant to have a third column with % republican votes.
The CORREL function will tell you an index of the strength of the relationship between the two variables, from -1 (strong negative relationship) to +1 (strong positive relationship). Close to 0 indicates little if any relationship.
If you square the result from CORREL, you get the coefficient of determination, or the percentage of variance in one variable "explained" by the variance in the other.
There are a whole slew of sites that go into regression / correlation in detail. A good one is HyperStat (ch 15):
http://davidmlane.com/hyperstat/