I am not aware of a possibility to get a list of the oldest caches in each of the counties.
However, what might be quite close to the result you want to achieve is to run a Map compare search with cache location "United States" and "Virginia" but leaving open the county field.
What you get is, beneath the map, a list with all caches in Virginia which is sorted by county and within the county by age. Strictly speaking by GC code, but most of the time the lower the code the earlier the cache.
There is one problem, though: as there are more than 10000 caches in Virginia, listing all the caches would be too much for the list. Therefore, you have to add a "hidden to" filter. You have to try around with the dates, I have put end of 2006 as an example:
https://project-gc.com/Tools/MapCompare?profile_name=k%2Bgw%2Ba&country=United+States®ion=Virginia&nonefound=on&onefound=on&hidden_toyyyy=2006&hidden_tomm=12&hidden_todd=31&submit=Filter. If some counties do not have any caches as old as that, fiddle around with the cut off date or check the (few?) missing counties manually.
Hope this helps. Happy new year!
