Averages are a bad way to look at age of car parks. A lot of people crash their cars in the first few years of ownership. Reasons are multiple, included but not limited to not knowing their car well, "trying out what it can do", first time young and inexperienced buyes, etc... For each 1 year old car that gets crashed (and thus scrapped), you will need a 25 year old car that gets scrapped. Or more realistically six 15 year old cars.... In this case the median age of a scrapped car would be much more interesting.
Also, "scrapped" is a bit of a misnomer. You read part about the cars I told you went to Africa? Those are scrapped cars. In the eyes of the Western World, they enter the stats as scrapped. Of course, in reality, that's not what happens to them in Africa.