Total cars on the road divided by new car sales. I seem to find useful pages only in German now (I am in Germany), which I expect wouldn't help you much, but s

Total cars on the road divided by new car sales. I seem to find useful pages only in German now (I am in Germany), which I expect wouldn't help you much, but statista.com and wikipedia.org has both, and if you wade through all the irrelevant graphs on statista you'll typically see that total/new is in the low teens for western countries (I checked three now, e.g. 47m/3.4m for .de, almost 14y lifetime or 7y average age).