Hey, just checked this topic again now. All of that is so crazy!
You might be interested to see this listing from late 2018.
https://www.auctionsinternational.com/auction/16233/item/1985-chevrolet-caprice-sedan-99961
Seems to have been in service right up until it was sold. This absolutely has to take the cake for the oldest anywhere.
As for the oldest now, there are at least three Caprices still around in Illinois at least in various roles.
1. Last I checked, McHenry County still has one.
2. There's a whale Caprice still in Waterman. (Taken 2019)
3. Niles also has one Caprice left, in public works service.
There's a box Crown Vic (a 1991, more specifically) in Tiburon, California.
https://www.google.com/maps/@37.8819605,-122.4662822,3a,25.6y,179.41h,69.57t/data=!3m6!1e1!3m4!1sNydl3FUwcgfRziUm-TwQaw!2e0!7i16384!8i8192
The paint is pretty trashed, but is otherwise in good shape.
It's weird, because they don't seem underfunded. They have new Chargers and Tauruses, so I'm not sure what the deal is with this. It can't be a show car if they're leaving it out in the elements. It's gotta be some sort of traffic unit. Could be like a chief's car as well.
There's a Caprice still in Nassau County, NY as well. This is still in service.
Lastly, there's an Aero Crown Vic with the San Francisco Sheriff's Department.