The average age of an American car is, at the moment, 14 years[1]. That means that there are about as many 28+ year old cars on the road as there are new cars.
Repairability becomes somewhat less relevant when reliability is better out-of-the-box.
Not to be too nit picky, but I think you’re conflating median and average. The median age is probably lower because the age distribution skews older due to vintage cars and such. But you are right about cars lasting much longer today. At the same time, I think there is a point that they are also less repairable. (I’ve heard horror stories of $7k+ touch screen replacements, which control everything from the radio to the HVAC).
Vintage cars are a tiny fraction of the vehicle base, and due to demand and population growth, and the fact that an old car had to have been a new car at some point, there is an immediate bias towards having more newer cars.
Also, unlike with money and wealth and other metrics where averages aren't very useful, the distribution of car ages does not have a tail of incredible outliers. There aren't a lot of billion-year-old cars driving that average away from the median.
Look, it's entirely possible that 'this time it'll be different', and we'll regress on this metric, but at the moment the data does not support it.
It doesn't compute like that because the minimum age is zero. So a long tail of fewer people driving stuff that's older than 28yr reduces the number of people driving things 14-28yr by greater than one each.
How does this square with your theory that cars are becoming 'more disposable'? They seem to be running longer than ever before.