http://www.usda.gov/nass/graphics/county03/crhar.htm
http://www.usda.gov/nass/graphics/county03/awhar.htm

???

I'm assuming factors like climate, soil content, water availability, and possibly optimal location for efficient distribution affect this, but why is it so concentrated in these areas?

http://images.google.ca/imgres?imgurl=http://www-atlas.usgs.gov/articles/people/IMAGES/65pop_fig3.gif&imgrefurl=http://www-atlas.usgs.gov/articles/people/a_age65pop.html&h=339&w=540&sz=109&hl=en&start=1&um=1&tbnid=96JuCNQNCppEPM:&tbnh=83&tbnw=132&prev=/images%3Fq%3Dpopulation%2Bmap%2Bunited%2Bstates%26 svnum%3D10%26um%3D1%26hl%3Den%26safe%3Doff%26clien t%3Dfirefox-a%26channel%3Ds%26rls%3Dorg.mozilla:en-US:official%26sa%3DN

heres a populaiton map that looks very similer. Its ages 65+, it was the first one that came up.

I didn't look at the links to be honest but a fair bit of intensive apple and stone fruit agriculture happens in my neck of the woods.

Things like the composition of the soil, the average rainfall, the distribution (being conveniently located for distribution/workers/services) I also would put it down to tradition??

You've already considered these though.