A car enthusiast is interested in studying the relationship between the asking price of used cars and their age. which is more naturally the explanatory variable?

The answer is Car's price.

In determining the price of a secod-hand cars, the basic variable that we could use is the initial price when the first owner of the car buy it.

Logically, the used price car's shall never be above the initial price of the car. And we can use maintenance duration, car's current condition, and additional acessory as adjustment variable.