The day could have been an outlier. There are several old car shows around Indiana, and there was one that took tons of cars on the state road near where we used to live, I guess because they didn’t want to use the interstate. The interstate is 65mph, and the state hwy is 55mph with a minimum of 40.
There are sports and muscle cars from before 1975, tons of cars from the 50s and 60s, 1940s roadsters, and the occasional luxury car from the 30s. There are older cars as well, but they are being dolly-towed. The median age of a car moving by its own engine is probably 1955, with the average being a little higher, maybe 1959, and the mode, I couldn’t say-- mid 50s for the sedan, and late 60s for the sports/muscle car, probably. Maybe even older, '48-'51 for luxury cars.
Now, I would wager that Indiana does, in fact, have one of the oldest mean ages for cars on the road, because we have no emissions controls on inspections whatsoever. I was driving a 1994 Integra until just 5 years ago, and now I am driving a 2002 Audi A4. I also drove a 1961 Falcon from about 1996-2006 (I sold it when I was pregnant, and I knew I wouldn’t have the time to work on it any more). My uncle drove a 1972 Chevelle up until 1997.
I don’t think there is any other state where those cars could be/have been on the road.
FWIW, you also see cars with spiderweb cracks on the windshield, a missing door with the hole covered by visqueen, missing hoods, different sized tires, missing grills, missing bumpers, missing rear windows, hand mirrors duct-taped over missing side mirrors. Cars held together by duct tape and bungee cords. Cars that take off in a cloud of black smoke. Cars dragging mufflers, and making sparks. You wouldn’t believe it if you live in California.