The mechanical usefulness life of a car is probably not linear. WELL MAINTAINED, it's probably a slow decline for the first 2-3 decades and then more rapid as parts reach the end of their lifespan and repairs cost more than the vehicle value. Cars end up at year, say 30, with 1 major issue but otherwise serviceable. But that issue is more expensive than the perceived value so it's parted out and scrapped. For instance, a old BMW might be worth $5000 with plenty of life, but has a $10,000 mechanical problem. Since few are willing to repair $10,000 on a $5000 car, it's scrapped. Hence, the utility value of a car is a slow decline until it often just drops off at the end.
And this probably explains why trucks hold more value longer. Cars are more pedestrian and when slightly worn or damaged, they still have the same utility but they drop with status/perceived value. Trucks are more robust but also generally expected to be worn or damaged so the owners don't care as much about paint or body damage. And truck owners tend to be more mechanical in nature and maintain them better.
The average age is around 12 years. Given millions of brand new cars are on the roads, that means there are a large % of 24 year old cars to offset that. Also, cash for clunkers really impacted that number, clearing a lot of aged cars off the roads. A real taxpayer subsidized boon for auto makers I'd say.
