Ppl typically bought new cars every 3-4 years back then. In '84 when I bought my 1st GS ('72) it was basically an old car although you could still walk into any parts store & get stuff for them & we drove them long distances. Today, my 2000 Toyota Camry doesn't seem that old really. The historic tag law here (MD) is totally abused because a 20 y/o car isn't that old so ppl buy 'em for DD's.
When I bought my 69 riviera in 84, it was only 15 years old but it looked old compared to new cars back then. It stuck out like a sore thumb in a parking lot. A 15 year old car today still looks contemporary.
40s and 50s cars.....but by the 60s things were better. I was racing my 67 Riv GS at 150K....original except for timing chain. Timing chains were not that hard to do compared to today. Alot of failed chains/sprockets sent cars right to junk yard. A buddy bought a 68 Caddy for $400 and we all 6 of us piled in and went to Nags Head at 90mph. Timing chain let go on the way home....I said I'd fix it for him....he junked it....