I want to say that the average age of a car in the mid to late 70s was 7-8 years.
That number started creeping up in the 80s and dont' think has ever really reversed.
One difference is the technology of the 60s and 70s was noticibly different to the motoring public compared to earlier decades. Our cars are pretty good. For the most part, even the basic models will still accelerate fast enough, stop quick enough, have all the lights and signals, etc that are needed to blend into traffic even on an interstate. Very few 40 year old cars in 1980 would have been able to do that comfortably. I think that is part of what relegated most of them to occassional use or the junk yard more quickly. Oh sure there's the exception. There was a Civil engineering teacher that drove his Model A into Newark 3-4 days a week (this was mid 1980s). But that was all local roads - I'm sure he didn't use the freeway.
